mtd: spi: SPI_FLASH_MTD depends on MTD

It is already the case that all defconfigs with SPI_FLASH_MTD also
declare using MTD, but let's make this consistent and enforce it in
Kconfig. Most of the time SPI_FLASH_MTD is used in conjunction with
UBI, which already depends on MTD.

Suggested-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>