dt-bindings: mailbox: Add a binding for the sun6i msgbox

This mailbox hardware is present in Allwinner sun6i, sun8i, sun9i, and
sun50i SoCs. Add a device tree binding for it.
Acked-by: NMaxime Ripard <mripard@kernel.org>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NSamuel Holland <samuel@sholland.org>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/allwinner,sun6i-a31-msgbox.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mailbox/allwinner,sun6i-a31-msgbox.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Allwinner sunxi Message Box
+
+maintainers:
+  - Samuel Holland <samuel@sholland.org>
+
+description: |
+  The hardware message box on sun6i, sun8i, sun9i, and sun50i SoCs is a
+  two-user mailbox controller containing 8 unidirectional FIFOs. An interrupt
+  is raised for received messages, but software must poll to know when a
+  transmitted message has been acknowledged by the remote user. Each FIFO can
+  hold four 32-bit messages; when a FIFO is full, clients must wait before
+  attempting more transmissions.
+
+  Refer to ./mailbox.txt for generic information about mailbox device-tree
+  bindings.
+
+properties:
+  compatible:
+    oneOf:
+      - items:
+          - enum:
+              - allwinner,sun8i-a83t-msgbox
+              - allwinner,sun8i-h3-msgbox
+              - allwinner,sun9i-a80-msgbox
+              - allwinner,sun50i-a64-msgbox
+              - allwinner,sun50i-h6-msgbox
+          - const: allwinner,sun6i-a31-msgbox
+      - const: allwinner,sun6i-a31-msgbox
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+    description: bus clock
+
+  resets:
+    maxItems: 1
+    description: bus reset
+
+  interrupts:
+    maxItems: 1
+
+  '#mbox-cells':
+    const: 1
+    description: first cell is the channel number (0-7)
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - resets
+  - interrupts
+  - '#mbox-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/sun8i-h3-ccu.h>
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    #include <dt-bindings/reset/sun8i-h3-ccu.h>
+
+    msgbox: mailbox@1c17000 {
+            compatible = "allwinner,sun8i-h3-msgbox",
+                         "allwinner,sun6i-a31-msgbox";
+            reg = <0x01c17000 0x1000>;
+            clocks = <&ccu CLK_BUS_MSGBOX>;
+            resets = <&ccu RST_BUS_MSGBOX>;
+            interrupts = <GIC_SPI 49 IRQ_TYPE_LEVEL_HIGH>;
+            #mbox-cells = <1>;
+    };
+
+...