dt-bindings: mailbox: Add Xilinx IPI Mailbox

Xilinx ZynqMP IPI(Inter Processor Interrupt) is a hardware block
in ZynqMP SoC used for the communication between various processor
systems.
Signed-off-by: NWendy Liang <wendy.liang@xilinx.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/xlnx,zynqmp-ipi-mailbox.txt

+Xilinx IPI Mailbox Controller
+========================================
+
+The Xilinx IPI(Inter Processor Interrupt) mailbox controller is to manage
+messaging between two Xilinx Zynq UltraScale+ MPSoC IPI agents. Each IPI
+agent owns registers used for notification and buffers for message.
+
+               +-------------------------------------+
+               | Xilinx ZynqMP IPI Controller        |
+               +-------------------------------------+
+    +--------------------------------------------------+
+ATF                    |                     |
+                       |                     |
+                       |                     |
+    +--------------------------+             |
+                       |                     |
+                       |                     |
+    +--------------------------------------------------+
+            +------------------------------------------+
+            |  +----------------+   +----------------+ |
+Hardware    |  |  IPI Agent     |   |  IPI Buffers   | |
+            |  |  Registers     |   |                | |
+            |  |                |   |                | |
+            |  +----------------+   +----------------+ |
+            |                                          |
+            | Xilinx IPI Agent Block                   |
+            +------------------------------------------+
+
+
+Controller Device Node:
+===========================
+Required properties:
+--------------------
+IPI agent node:
+- compatible:		Shall be: "xlnx,zynqmp-ipi-mailbox"
+- interrupt-parent:	Phandle for the interrupt controller
+- interrupts:		Interrupt information corresponding to the
+			interrupt-names property.
+- xlnx,ipi-id:		local Xilinx IPI agent ID
+- #address-cells:	number of address cells of internal IPI mailbox nodes
+- #size-cells:		number of size cells of internal IPI mailbox nodes
+
+Internal IPI mailbox node:
+- reg:			IPI buffers address ranges
+- reg-names:		Names of the reg resources. It should have:
+			* local_request_region
+			  - IPI request msg buffer written by local and read
+			    by remote
+			* local_response_region
+			  - IPI response msg buffer written by local and read
+			    by remote
+			* remote_request_region
+			  - IPI request msg buffer written by remote and read
+			    by local
+			* remote_response_region
+			  - IPI response msg buffer written by remote and read
+			    by local
+- #mbox-cells:		Shall be 1. It contains:
+			* tx(0) or rx(1) channel
+- xlnx,ipi-id:		remote Xilinx IPI agent ID of which the mailbox is
+			connected to.
+
+Optional properties:
+--------------------
+- method:              The method of accessing the IPI agent registers.
+                       Permitted values are: "smc" and "hvc". Default is
+                       "smc".
+
+Client Device Node:
+===========================
+Required properties:
+--------------------
+- mboxes:		Standard property to specify a mailbox
+			(See ./mailbox.txt)
+- mbox-names:		List of identifier  strings for each mailbox
+			channel.
+
+Example:
+===========================
+	zynqmp_ipi {
+		compatible = "xlnx,zynqmp-ipi-mailbox";
+		interrupt-parent = <&gic>;
+		interrupts = <0 29 4>;
+		xlnx,ipi-id = <0>;
+		#address-cells = <1>;
+		#size-cells = <1>;
+		ranges;
+
+		/* APU<->RPU0 IPI mailbox controller */
+		ipi_mailbox_rpu0: mailbox@ff90400 {
+			reg = <0xff990400 0x20>,
+			      <0xff990420 0x20>,
+			      <0xff990080 0x20>,
+			      <0xff9900a0 0x20>;
+			reg-names = "local_request_region",
+				    "local_response_region",
+				    "remote_request_region",
+				    "remote_response_region";
+			#mbox-cells = <1>;
+			xlnx,ipi-id = <1>;
+		};
+		/* APU<->RPU1 IPI mailbox controller */
+		ipi_mailbox_rpu1: mailbox@ff990440 {
+			reg = <0xff990440 0x20>,
+			      <0xff990460 0x20>,
+			      <0xff990280 0x20>,
+			      <0xff9902a0 0x20>;
+			reg-names = "local_request_region",
+				    "local_response_region",
+				    "remote_request_region",
+				    "remote_response_region";
+			#mbox-cells = <1>;
+			xlnx,ipi-id = <2>;
+		};
+	};
+	rpu0 {
+		...
+		mboxes = <&ipi_mailbox_rpu0 0>,
+			 <&ipi_mailbox_rpu0 1>;
+		mbox-names = "tx", "rx";
+	};
+	rpu1 {
+		...
+		mboxes = <&ipi_mailbox_rpu1 0>,
+			 <&ipi_mailbox_rpu1 1>;
+		mbox-names = "tx", "rx";
+	};