rockchip: rk3188: Add core devicetree files

The rk3188 shares a lot of peripherals with the rk3066 and thus
has a common include called rk3xxx.dtsi. Add both this one and
the specialized rk3188 on top of it.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>