mtd: rawnand: gpio: Implement exec_op()

Implement exec_op() so we can get rid of the legacy interface
implementation.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200603150746.1423257-3-boris.brezillon@collabora.com