[media] cobalt: add MTD dependency

The cobalt driver fails to link when it is built-in and MTD is disabled or a
loadable module:

drivers/media/built-in.o: In function `cobalt_flash_probe':
:(.text+0xb8b46): undefined reference to `mtd_device_parse_register'
:(.text+0xb8b88): undefined reference to `do_map_probe'
drivers/media/built-in.o: In function `cobalt_flash_remove':
:(.text+0xb8bb4): undefined reference to `mtd_device_unregister'
:(.text+0xb8bbe): undefined reference to `map_destroy'

This adds a Kconfig dependency to ensure we can call the API.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>