arm: dts: rk3036: Add rk3036-u-boot.dtsi

Add U-Boot specific dtsi file for rk3036 SoC. This
would help to add U-Boot specific dts nodes, properties
which are common across rk3036.

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>