dt-bindings: mtd: Document ARASAN NAND bindings

Document the Arasan NAND controller bindings.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20200519074549.23673-7-miquel.raynal@bootlin.com