dt-bindings: mtd: Document Macronix NAND device bindings

Document the bindings used by the Macronix NAND device.

Signed-off-by: Mason Yang <masonccyang@mxic.com.tw>
Reviewed-by: Rob Herring <robh@kernel.org>
[<miquel.raynal@bootlin.com: Fix typos]
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1581922600-25461-3-git-send-email-masonccyang@mxic.com.tw