dt-bindings: riscv: microchip: Add YAML documentation for the PolarFire SoC

Add YAML DT binding documentation for the Microchip PolarFire SoC.
It is documented at:

https://www.microsemi.com/products/fpga-soc/polarfire-soc-icicle-quick-start-guideSigned-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

Documentation/devicetree/bindings/riscv/microchip.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/riscv/microchip.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Microchip PolarFire SoC-based boards device tree bindings
+
+maintainers:
+  - Cyril Jean <Cyril.Jean@microchip.com>
+  - Lewis Hanly <lewis.hanly@microchip.com>
+
+description:
+  Microchip PolarFire SoC-based boards
+
+properties:
+  $nodename:
+    const: '/'
+  compatible:
+    items:
+      - enum:
+          - microchip,mpfs-icicle-kit
+      - const: microchip,mpfs
+
+additionalProperties: true
+
+...