mtd: ensure MTD is compiled when CMD_MTDPARTS is selected

MTD support must be enabled when using mtdparts. Indeed, functions
like get_mtd_info(), get_mtd_device() and put_mtd_device() are in
drivers/mtd/mtd_uboot.c and are built only with CONFIG_MTD.

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