mtd: always build partitioning support

There are very few situations where MTD partitioning is not required,
and the benefit in code size reduction by making this configurable does
not warrant the level of ifdeffery needed.

Artem: this patch is not final - we just make sure that mtd partitions
are always compiled in, and at the end of the series we'll kill
MTD_PARTITIONS altogether.

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>