dt-bindings: add bindings for polarfire soc system controller

Add device tree bindings for the MSS system controller 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/soc/microchip/microchip,polarfire-soc-sys-controller.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/soc/microchip/microchip,polarfire-soc-sys-controller.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Microchip PolarFire SoC (MPFS) MSS (microprocessor subsystem) system controller
+
+maintainers:
+  - Conor Dooley <conor.dooley@microchip.com>
+
+description: |
+  The PolarFire SoC system controller is communicated with via a mailbox.
+  This document describes the bindings for the client portion of that mailbox.
+
+
+properties:
+  mboxes:
+    maxItems: 1
+
+  compatible:
+    const: microchip,polarfire-soc-sys-controller
+
+required:
+  - compatible
+  - mboxes
+
+additionalProperties: false
+
+examples:
+  - |
+    syscontroller: syscontroller {
+      compatible = "microchip,polarfire-soc-sys-controller";
+      mboxes = <&mbox 0>;
+    };