board: rockchip: rgxx3: Use sdmmc0 as first device

Some of the rgxx3 devices do not have a way to recover from a poor
flash of a bootloader to eMMC. Set the device to always attempt to boot
from sdmmc0 first which ensures that we can override the boot from
emmc if we have a card present with a valid fit signature. The
expectation is that this will protect from the very unlikely chance
we have a valid FIT signature on the eMMC but the U-Boot stage fails
for some other reason.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>