rockchip: dts: rk3399: Create initial rk3399-u-boot.dtsi

u-boot,dm-pre-reloc is required for SDMMC booted rk3399 boards and
which is U-Boot specific devicetrees binding.

Move it on global rk3399-u-boot.dtsi file and rest of the U-Boot
bindings will move it future based on the requirement.

This would help to sync the devicetrees from Linux whenever required
instead of adding specific nodes.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>