arm: mvebu: spl: Load proper U-Boot from selected eMMC boot partition

When eMMC boot is selected then BootROM loads kwbimage header (U-Boot SPL)
from the selected eMMC boot partition. So for eMMC boot ensure that U-Boot
SPL loads U-Boot proper (from kwbimage) also from the same selected eMMC
boot partition.

Fixes: 2226ca17 ("arm: mvebu: Load U-Boot proper binary in SPL code based on kwbimage header")
Signed-off-by: Pali Rohár <pali@kernel.org>