mtd: rawnand: Deprecate the ->select_chip() hook

Now that the CS line to be selected is passed to ->exec_op() and
stored in chip->cur_cs and after patching all drivers implementing
->exec_op() to stop implementing this method, we can deprecate it by
moving it to the nand_legacy structure.

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>