arm64: dts: rockchip: Support poweroff on NanoPC-T6

The RK806 on the NanoPC-T6 can be used to power on/off the whole board.
Mark it as the system power controller.
Signed-off-by: Hugh Cole-Baker <sigmaris@gmail.com>
Link: https://lore.kernel.org/r/20231216212134.23314-1-sigmaris@gmail.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3588-nanopc-t6.dts

@@ -569,6 +569,8 @@ pmic@0 {
 		pinctrl-0 = <&pmic_pins>, <&rk806_dvs1_null>,
 			    <&rk806_dvs2_null>, <&rk806_dvs3_null>;
 
+		system-power-controller;
+
 		vcc1-supply = <&vcc4v0_sys>;
 		vcc2-supply = <&vcc4v0_sys>;
 		vcc3-supply = <&vcc4v0_sys>;