dt-bindings: power: reset: xilinx: Add bindings for ipi mailbox

Add IPI mailbox property and its example in xilinx zynqmp-power
documentation.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

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

@@ -8,9 +8,41 @@ Required properties:
  - compatible:		Must contain:	"xlnx,zynqmp-power"
  - interrupts:		Interrupt specifier
 
--------
-Example
--------
+Optional properties:
+ - mbox-names	: Name given to channels seen in the 'mboxes' property.
+		  "tx" - Mailbox corresponding to transmit path
+		  "rx" - Mailbox corresponding to receive path
+ - mboxes	: Standard property to specify a Mailbox. Each value of
+		  the mboxes property should contain a phandle to the
+		  mailbox controller device node and an args specifier
+		  that will be the phandle to the intended sub-mailbox
+		  child node to be used for communication. See
+		  Documentation/devicetree/bindings/mailbox/mailbox.txt
+		  for more details about the generic mailbox controller
+		  and client driver bindings. Also see
+		  Documentation/devicetree/bindings/mailbox/ \
+		  xlnx,zynqmp-ipi-mailbox.txt for typical controller that
+		  is used to communicate with this System controllers.
+
+--------
+Examples
+--------
+
+Example with interrupt method:
+
+firmware {
+	zynqmp_firmware: zynqmp-firmware {
+		compatible = "xlnx,zynqmp-firmware";
+		method = "smc";
+
+		zynqmp_power: zynqmp-power {
+			compatible = "xlnx,zynqmp-power";
+			interrupts = <0 35 4>;
+		};
+	};
+};
+
+Example with IPI mailbox method:
 
 firmware {
 	zynqmp_firmware: zynqmp-firmware {
@@ -19,7 +51,11 @@ firmware {
 
 		zynqmp_power: zynqmp-power {
 			compatible = "xlnx,zynqmp-power";
+			interrupt-parent = <&gic>;
 			interrupts = <0 35 4>;
+			mboxes = <&ipi_mailbox_pmu0 0>,
+				 <&ipi_mailbox_pmu0 1>;
+			mbox-names = "tx", "rx";
 		};
 	};
 };