davinci: omapl138_lcdk: add u-boot sector for mmc/sd boot

Set the correct CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR value in order
to be able to boot from MMC/SD.

The SPL is stored at sector 0x75, while u-boot will follow at
sector 0xb5.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>