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

devm_fwnode_get_index_gpiod_from_child() is going away as the name is
too unwieldy, let's switch to using the new devm_fwnode_gpiod_get().

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>