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

drivers/mtd/nand/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
+
+menu "NAND"
+
 config MTD_NAND_CORE
 	tristate
 
 source "drivers/mtd/nand/onenand/Kconfig"
 source "drivers/mtd/nand/raw/Kconfig"
 source "drivers/mtd/nand/spi/Kconfig"
+
+endmenu