arm64: rockchip: dts: rk3399: Add board -u-boot.dtsi files

Devicetree files in RK3399 platform is synced from Linux, like other
platforms does. Apart from these u-boot in rk3399 would also require
some u-boot specific node like dmc.

dmc node has big chunk of DDR timing parameters which are specific
to specific board, and maintained with rk3399-sdram*.dtsi.

So, create board specific -u-boot.dtsi files and move these sdram dtsi
files accordingly. This would help of maintain u-boot specific changes
separately without touching Linux dts(i) files which indeed easy for
syncing from Linux between releases.

These board specific -u-boot.dtsi can be extendible to add more u-boot
specific nodes or properties in future.

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