rockchip: px30: insert u-boot, spl-boot-device into U-Boot device tree

It is possible to boot U-Boot proper from a different storage medium
than the one used by the BOOTROM to load the SPL. This information is
stored in the u-boot,spl-boot-device Device Tree property and is
accessible from U-Boot proper so that it has knowledge at runtime where
it was loaded from.

Let's add support for this feature for px30.

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>