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 <festevam@denx.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>