mtd: rawnand: makes the Kconfig entry clear when it comes to raw NANDs

Files have been moved in the NAND subsystem to reflect the different
flavors of NAND devices.

Raw/Parallel NAND devices have been moved to a "raw" subdirectory to
make the difference with OneNAND and SPI NAND for instance. So adjust
the Kconfig entry to clarify things.

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