mtd: declare MTD_PARTITIONS symbol in Kconfig

UBI selects MTD_PARTITIONS which is the symbol to compile
drivers/mtd/mtdpart.c. Unfortunately, the symbol was not defined in
Kconfig and this worked only with board files defining it. Fix this by
adding a boolean in Kconfig so boards defined by defconfig files only
will work as expected.

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