mtd: rename CONFIG_NAND -> CONFIG_MTD_RAW_NAND

Add more clarity by changing the Kconfig entry name.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
[trini: Re-run migration, update a few more cases]
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>