MMC: Enable DM_MMC for Davinci

With CONFIG_BLK becoming a requirement, the Davinci MMC driver
needs to be updated with DM_MMC support.  Since SPL is tiny and
many boards do not support DM in SPL, this retains the backwards
compatibility for those boards who need to initialize MMC manually
in SPL.

Signed-off-by: Peter Howard <phoward@gme.net.au>
Signed-off-by: Adam Ford <aford173@gmail.com>