mtd: rawnand: Add nand_[de]select_target() helpers

Add a wrapper to prevent drivers and core code from directly calling
the ->select_chip hook which we are about to deprecate.

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>