dt-bindings: mtd: mxc-nand: Document fsl,imx31-nand

imx31.dtsi uses the following NAND compatible:

compatible = "fsl,imx31-nand", "fsl,imx27-nand";

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

compatible: ['fsl,imx31-nand', 'fsl,imx27-nand'] is too long

Signed-off-by: Fabio Estevam <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>

Documentation/devicetree/bindings/mtd/mxc-nand.yaml

···
 
 properties:
   compatible:
-    const: fsl,imx27-nand
-
+    oneOf:
+      - const: fsl,imx27-nand
+      - items:
+          - enum:
+              - fsl,imx31-nand
+          - const: fsl,imx27-nand
   reg:
     maxItems: 1