dt-bindings: add bindings for polarfire soc mailbox

Add device tree bindings for the MSS system controller mailbox on
the Microchip PolarFire SoC.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/microchip,polarfire-soc-mailbox.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/mailbox/microchip,polarfire-soc-mailbox.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Microchip PolarFire SoC (MPFS) MSS (microprocessor subsystem) mailbox controller
+
+maintainers:
+  - Conor Dooley <conor.dooley@microchip.com>
+
+properties:
+  compatible:
+    const: microchip,polarfire-soc-mailbox
+
+  reg:
+    items:
+      - description: mailbox data registers
+      - description: mailbox interrupt registers
+
+  interrupts:
+    maxItems: 1
+
+  "#mbox-cells":
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - "#mbox-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    soc {
+      #address-cells = <2>;
+      #size-cells = <2>;
+      mbox: mailbox@37020000 {
+        compatible = "microchip,polarfire-soc-mailbox";
+        reg = <0x0 0x37020000 0x0 0x1000>, <0x0 0x2000318c 0x0 0x40>;
+        interrupt-parent = <&L1>;
+        interrupts = <96>;
+        #mbox-cells = <1>;
+      };
+    };