mtd: rawnand: davinci: Implement exec_op()

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

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200513172248.141402-3-boris.brezillon@collabora.com