mtd: rawnand: Make ->select_chip() optional when ->exec_op() is implemented

Now that the CS to be selected on a nand_operation is passed in
nand_operation->cs we can make the ->select_chip() hook optional for
drivers implementing ->exec_op().

When not implemented, the core is assuming the CS line is automatically
asserted/deasserted by the driver ->exec_op() implementation.

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>