mmc: add mmc hwpartition sub-command to do eMMC hardware partitioning

Adds the mmc hwpartition sub-command to perform eMMC hardware
partitioning on an mmc device. The number of arguments can be
large for a complex partitioning, but as the partitioning has
to be done in one go it is difficult to make it simpler.

Signed-off-by: Diego Santa Cruz <Diego.SantaCruz@spinetix.com>