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

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

@@ -114,6 +114,13 @@ patternProperties:
         description:
           Contains the native Ready/Busy IDs.
 
+      rb-gpios:
+        description:
+          Contains one or more GPIO descriptor (the numper of descriptor
+          depends on the number of R/B pins exposed by the flash) for the
+          Ready/Busy pins. Active state refers to the NAND ready state and
+          should be set to GPIOD_ACTIVE_HIGH unless the signal is inverted.
+
     required:
       - reg