ARM: dts: rockchip: Add rk3588-u-boot.dtsi

Add u-boot,dm-spl and u-boot,dm-pre-reloc related properties
for Rockchip RK3588 SoC to boot the SPL.

Signed-off-by: Jagan Teki <jagan@edgeble.ai>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>