rockchip: rk3308: fix same-as-spl boot order

Rockchip SoCs need the boot_devices array defined in order to map the
bootloader's value to a U-Boot device.  Implement this for rk3308.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>