rockchip: rk3188: add -u-boot.dtsi for rock-rk3188

We should a -u-boot.dtsi for those config need by U-Boot only,
and other part sync with kernel.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>