dt-bindings: add bindings for polarfire soc system controller

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+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/soc/microchip/microchip,mpfs_sys_controller.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Microchip MPFS system controller
+
+maintainers:
+  - Conor Dooley <[email protected]>
+
+properties:
+  compatible:
+    const: microchip,polarfire-soc-sys-controller # PolarFire
+
+  mbox-names:
+    maxItems: 1
+    description: name of the mailbox controller device node
+
+  mboxes:
+    maxItems: 1
+    description: |
+      phandle and index of the mailbox controller device node. It must be 0 (hardware supports only one channel).
+
+
+  "#address-cells":
+    const: 1
+
+  "#size-cells":
+    const: 1
+
+required:
+  - compatible
+  - mbox-names
+  - "#address-cells"
+  - "#size-cells"
+  - "mboxes"
+
+unevaluatedProperties: false
+additionalProperties: false
+
+examples:
+  - |
+    syscontroller@37020000 {
+      compatible = "microchip,polarfire-soc-sys-controller";
+      #address-cells = <1>;
+      #size-cells = <1>;
+      mbox-names = "mbox-mpfs";
+      mboxes = <&mbox 0>;
+    };