ARM: dts: rockchip: add cpu supplies to boards

This adds the cpu-supplies to the curie2, radxarock and rk3288-evb-rk808 boards.
The rk3288-evb-act8846 variant cannot provide the cpu-regulator at the moment
so will get its supply later.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>