dt-bindings: Add documentation for Milbeaut SoCs

This adds a DT binding documentation for the M10V and its evaluation
board.
Signed-off-by: Sugaya Taichi <sugaya.taichi@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/arm/socionext/milbeaut.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/milbeaut.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Milbeaut platforms device tree bindings
+
+maintainers:
+  - Taichi Sugaya <sugaya.taichi@socionext.com>
+  - Takao Orito <orito.takao@socionext.com>
+
+properties:
+  $nodename:
+    const: '/'
+  compatible:
+    oneOf:
+      - items:
+          - enum:
+              - socionext,milbeaut-m10v-evb
+          - const: socionext,sc2000a
+...