arm: mvebu: fix boot from UART when in fallback mode

It's the first 8 bits of the bootrom error register that
contain the boot error/fallback error code. Let's check that
and continue to boot from UART.

Signed-off-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Signed-off-by: Stefan Roese <sr@denx.de>