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

Author: Christophe Kerello

Documentation/devicetree/bindings/mtd/nand-controller.yaml

@@ -154,6 +154,13 @@ patternProperties:
           Ready/Busy pins. Active state refers to the NAND ready state and
           should be set to GPIOD_ACTIVE_HIGH unless the signal is inverted.
 
+      wp-gpios:
+        description:
+          Contains one GPIO descriptor for the Write Protect pin.
+          Active state refers to the NAND Write Protect state and should be
+          set to GPIOD_ACTIVE_LOW unless the signal is inverted.
+        maxItems: 1
+
       secure-regions:
         $ref: /schemas/types.yaml#/definitions/uint64-matrix
         description: