mtd: rawnand: atmel: Stop including gpio.h

gpio/consumer.h defines everything we need, and it's clearly stated in
gpio.h that GPIO consumers should directly stop including gpio.h if they
can.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>