dt-binding: mtd: nand: Document the wp-gpios property

A few drivers use this property to describe the GPIO pin used to protect
the NAND during program/erase operations.

Signed-off-by: Christophe Kerello <christophe.kerello@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220217144755.270679-2-christophe.kerello@foss.st.com