ARM: davinci: omapl138_lcdk: Enable DM_MMC

With DM_MMC now available, this patch enables DM_MMC for the
omapl138_lcdk in U-Boot and keeps the older style for SPL.

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