mtd: provide of_mtd_parse_partitions for !CONFIG_MTD_OF_PARTS

If we don't have OpenFirmware enabled then provide a stub
of_mtd_parse_partitions that returns no partitions so drivers don't need
ifdeffery inside.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>