mtd: rawnand: Provide helper for polling GPIO R/B pin
Each controller driver having access to NAND R/B pin over GPIO would have to reimplement the polling loop otherwise. Suggested-by: Boris Brezillon <boris.brezillon@bootlin.com> Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com> Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com> Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>