mmc: add API to do eMMC hardware partitioning

This adds an API to do hardware partitioning on eMMC devices. The
new mmc_hwpart_config() function does the partitioning in one go.
As the different attributes and partitioning options on eMMC may
be interdependent validation has to be done based on the complete
partitioning configuration. The function accepts three modes:

- MMC_HWPART_CONF_CHECK: just validates that the configuration
  is valid.
- MMC_HWPART_CONF_SET: validates and sets all the fields in
  EXT_CSD but without setting the "partitioning completed" bit,
  and thus is reversible.
- MMC_HWPART_CONF_COMPLETE: does everything and is thus not
  reversible.

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