arm: dts: rk3188: Add rk3188-u-boot.dtsi

Add U-Boot specific dtsi file for rk3188 SoC. This
would help to add U-Boot specific dts nodes, properties
which are common across rk3188.

Right now, the file is empty, will add required changes
in future patches.

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