arm: mvebu: clearfog: set uboot image SD card offset

arm: mvebu: clearfog: set uboot image SD card offset

Armada 38x ROM skips the first SD card offset when loading SPL. This
affects the location of the main U-Boot image. SPL MMC code now supports
U-Boot image offset based on run-time detection of the boot partition.
Use this feature to make the same generated image support both SD card
and eMMC boot partition.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>