mtd: rawnand: mxc: implement exec_op

This converts the driver to the more modern exec_op which gets us rid
of a bunch of legacy code. Tested on i.MX27 and i.MX25.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240522-mtd-nand-mxc-nand-exec-op-v4-2-75b611e0ac44@pengutronix.de