dt-bindings: rtc: Add fsl,scu-rtc 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 'rtc' 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/arm/freescale/fsl,scu.txt

@@ -62,12 +62,6 @@ i.MX SCU Client Device Node:
 
 Client nodes are maintained as children of the relevant IMX-SCU device node.
 
-RTC bindings based on SCU Message Protocol
-------------------------------------------------------------
-
-Required properties:
-- compatible: should be "fsl,imx8qxp-sc-rtc";
-
 Watchdog bindings based on SCU Message Protocol
 ------------------------------------------------------------
 
@@ -116,10 +110,6 @@ firmware {
 			  &lsio_mu1 1 3
 			  &lsio_mu1 3 3>;
 
-		rtc: rtc {
-			compatible = "fsl,imx8qxp-sc-rtc";
-		};
-
 		watchdog {
 			compatible = "fsl,imx8qxp-sc-wdt", "fsl,imx-sc-wdt";
 			timeout-sec = <60>;

Documentation/devicetree/bindings/rtc/fsl,scu-rtc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/rtc/fsl,scu-rtc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: i.MX SCU Client Device Node - RTC 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: rtc.yaml#
+
+properties:
+  compatible:
+    const: fsl,imx8qxp-sc-rtc
+
+required:
+  - compatible
+
+additionalProperties: false
+
+examples:
+  - |
+    rtc {
+        compatible = "fsl,imx8qxp-sc-rtc";
+    };