ARM: dts: rockchip: add system-power-controller to act8846 on radxarock

The act8846 is the main pmic and system-power-controller on radxarock boards,
so add the necessary property.

Signed-off-by: Michael Niewoehner <mniewoeh@stud.hs-offenburg.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>