mtd: rawnand: stm32_fmc2: switch to using devm_fwnode_gpiod_get()

I would like to stop exporting OF-specific devm_gpiod_get_from_of_node()
so that gpiolib can be cleaned a bit, so let's switch to the generic
fwnode property API.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220903-gpiod_get_from_of_node-remove-v1-3-b29adfb27a6c@gmail.com