mtd: rawnand: Get rid of chip->bits_per_cell

Now that we inherit from nand_device, we can use
nand_device->memorg.bits_per_cell instead of having our own field at
the nand_chip level.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>