ARM: dts: rockchip: update rk3188-radxarock.dts

In the Linux DT the file rk3xxx.dtsi is shared between
rk3066 and rk3188. Both rk3xxx.dtsi and rk3188.dtsi have recently
had some updates.
For a future rk3066 support in U-boot this file must also update.
Move U-boot specific things in a rk3188-radxarock-u-boot.dtsi file.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>