dt-bindings: soc: Add ZynqMP PM bindings

Add documentation to describe Xilinx ZynqMP power management
bindings.
Signed-off-by: NRajan Vaja <rajan.vaja@xilinx.com>
Signed-off-by: NJolly Shah <jolly.shah@xilinx.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

Documentation/devicetree/bindings/power/reset/xlnx,zynqmp-power.txt

+--------------------------------------------------------------------
+Device Tree Bindings for the Xilinx Zynq MPSoC Power Management
+--------------------------------------------------------------------
+The zynqmp-power node describes the power management configurations.
+It will control remote suspend/shutdown interfaces.
+
+Required properties:
+ - compatible:		Must contain:	"xlnx,zynqmp-power"
+ - interrupts:		Interrupt specifier
+
+-------
+Example
+-------
+
+firmware {
+	zynqmp_firmware: zynqmp-firmware {
+		compatible = "xlnx,zynqmp-firmware";
+		method = "smc";
+
+		zynqmp_power: zynqmp-power {
+			compatible = "xlnx,zynqmp-power";
+			interrupts = <0 35 4>;
+		};
+	};
+};