ARM: dts: rockchip: set system-power-controller property on rk3288-rock2

The act8846 on rock2 boards controls the system power, so add the
appropriate property to allow turning off the board.
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm/boot/dts/rk3288-rock2-som.dtsi

@@ -109,6 +109,7 @@ &i2c0 {
 	act8846: act8846@5a {
 		compatible = "active-semi,act8846";
 		reg = <0x5a>;
+		system-power-controller;
 		inl1-supply = <&vcc_io>;
 		inl2-supply = <&vcc_sys>;
 		inl3-supply = <&vcc_20>;