mtd: rawnand: fsl_upm: Use gpio descriptors

The integer-based GPIO ids are now deprecated in favor of the GPIO desc
API. The PPC platforms have already been converted to GPIOLIB, so let's
use gpio descs in the NAND driver too.

While at it, we use devm_gpiod_get_index_optional() so we can get rid
of the manual gpio desc release done in the init error path and in the
remove function.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200603134922.1352340-7-boris.brezillon@collabora.com