dt-bindings: mtd: ti,gpmc-nand: Add missing 'rb-gpios'

With 'unevaluatedProperties' support implemented, the TI GPMC example
has a warning:

Documentation/devicetree/bindings/memory-controllers/ti,gpmc.example.dt.yaml: nand@0,0: Unevaluated properties are not allowed ('rb-gpios' was unexpected)

Add the missing definition for 'rb-gpios'.

Cc: Miquel Raynal <[e-mail]>
Cc: Richard Weinberger <[e-mail]>
Cc: Vignesh Raghavendra <[e-mail]>
Cc: Tony Lindgren <[e-mail]>
Cc: Roger Quadros <[e-mail]>
Cc: [e-mail]
Signed-off-by: Rob Herring <[e-mail]>
Reviewed-by: Thierry Reding <[e-mail]>
Reviewed-by: Roger Quadros <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

Documentation/devicetree/bindings/mtd/ti,gpmc-nand.yaml

···
     enum: [8, 16]
     default: 8
 
+  rb-gpios:
+    description:
+      GPIO connection to R/B signal from NAND chip
+    maxItems: 1
+
 patternProperties:
   "@[0-9a-f]+$":
     $ref: "/schemas/mtd/partitions/partition.yaml"