dt-bindings: mtd: nand: Document the generic rb-gpios property

A few drivers use this property to describe GPIO pins used to sample
the NAND Ready/Busy state. Let's make it part of the generic binding
doc.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200519232454.374081-2-boris.brezillon@collabora.com