mtd: nand: Add an extra level in the Kconfig hierarchy

Use an extra level in Kconfig for all NAND related entries.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529002517.3546-11-miquel.raynal@bootlin.com