mtd: rawnand: Deprecate the dummy_controller field

We try to force NAND controller drivers to properly separate the NAND
controller object from the NAND chip one, so let's deprecate the dummy
controller object embedded in nand_chip to encourage them to create
their own instance.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>