ubi: Depend on MTD

UBI required MTD to build correctly, add it as a Kconfig dependency.

Link: https://lore.kernel.org/all/20240411-mtd-v1-1-fe300f6ab657@jookia.org


Signed-off-by: John Watts <contact@jookia.org>
Reviewed-by: Michael Trimarchi <michael@amarulasolutins.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>