da850/omap-l138: Add support to read u-boot image from MMC/SD

da850/omap-l138: Add support to read u-boot image from MMC/SD

DA850/OMAP-L138 does not support strict MMC/SD boot mode. SPL will
be in SPI flash and U-Boot image will be in MMC/SD card. SPL will
do the low level initialization and then loads the u-boot image
from MMC/SD card.

Define CONFIG_SPL_MMC_LOAD macro in the DA850/OMAP-L138
configuration file to enable this feature.

Tested-by: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Signed-off-by: Rajashekhara, Sudhakar <sudhakar.raj@ti.com>
Signed-off-by: Hadli, Manjunath <manjunath.hadli@ti.com>