rockchip: px30: list possible SPL boot devices

BOOTROM sets a bit in a CPU register so that the software can know from
where the first stage bootloader was booted. One use case for this is to
specify the default loading medium for U-Boot proper to match the one
used by the BOOTROM to load the SPL (same-as-spl in
u-boot,spl-boot-order).

Let's create the mapping between BOOTROM value and Device Tree node
names for MMC devices.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>