rockchip: rk3588: Add boot device detection

Enable SPL on RK3588 to detect which device it was booted from.
Fixes use of same-as-spl in u-boot,spl-boot-order prop.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>