dt-bindings: rtc: mxc: Document fsl,imx31-rtc

imx31.dtsi uses the following RTC compatible:

compatible = "fsl,imx31-rtc", "fsl,imx21-rtc";

Document 'fsl,imx31-rtc' to fix the following dt-schema warning:

'fsl,imx31-rtc' is not one of ['fsl,imx1-rtc', 'fsl,imx21-rtc']

Signed-off-by: Fabio Estevam <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexandre Belloni <[e-mail]>

Documentation/devicetree/bindings/rtc/rtc-mxc.yaml

···
 
 properties:
   compatible:
-    enum:
-      - fsl,imx1-rtc
-      - fsl,imx21-rtc
+    oneOf:
+      - const: fsl,imx1-rtc
+      - const: fsl,imx21-rtc
+      - items:
+          - enum:
+              - fsl,imx31-rtc
+          - const: fsl,imx21-rtc
 
   reg:
     maxItems: 1