dt-bindings: zynqmp: add destination mailbox compatible

Current dt-bindings does not contain compatible property
for child mailbox node. Child mailbox nodes are used
to represent destination (remote) IPI agents. The compatible
property for all destination mailboxes must be required to
identify the node.

This addition of new required property does not break ABI
as current driver does not use this compatible property
but, new features in driver should use this property.
Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

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

@@ -74,6 +74,10 @@ patternProperties:
     type: object  # DT nodes are json objects
     additionalProperties: false
     properties:
+
+      compatible:
+        const: xlnx,zynqmp-ipi-dest-mailbox
+
       xlnx,ipi-id:
         description:
           Remote Xilinx IPI agent ID of which the mailbox is connected to.
@@ -95,6 +99,7 @@ patternProperties:
           - const: remote_response_region
 
     required:
+      - compatible
       - reg
       - reg-names
       - "#mbox-cells"
@@ -124,6 +129,7 @@ examples:
         ranges;
 
         mailbox: mailbox@ff9905c0 {
+          compatible = "xlnx,zynqmp-ipi-dest-mailbox";
           reg = <0x0 0xff9905c0 0x0 0x20>,
                 <0x0 0xff9905e0 0x0 0x20>,
                 <0x0 0xff990e80 0x0 0x20>,