mtd: rawnand: Move the ->exec_op() method to nand_controller_ops

->exec_op() is a controller method and has nothing to do in the
nand_chip struct. Let's move it to the nand_controller_ops struct and
adjust the core and drivers accordingly.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>