dt-bindings: input: Add fsl,scu-key yaml file

In order to replace the fsl,scu txt file from bindings/arm/freescale,
we need to split it between the right subsystems. This patch documents
separately the 'keys' child node of the SCU main node.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/input/fsl,scu-key.yaml

@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/input/fsl,scu-key.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: i.MX SCU Client Device Node - SCU key bindings based on SCU Message Protocol
+
+maintainers:
+  - Dong Aisheng <aisheng.dong@nxp.com>
+
+description: i.MX SCU Client Device Node
+  Client nodes are maintained as children of the relevant IMX-SCU device node.
+
+allOf:
+  - $ref: input.yaml#
+
+properties:
+  compatible:
+    items:
+      - const: fsl,imx8qxp-sc-key
+      - const: fsl,imx-sc-key
+
+  linux,keycodes:
+    maxItems: 1
+
+required:
+  - compatible
+  - linux,keycodes
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/input/input.h>
+
+    keys {
+        compatible = "fsl,imx8qxp-sc-key", "fsl,imx-sc-key";
+        linux,keycodes = <KEY_POWER>;
+    };