ARM: dts: rockchip: enable saradc for rk3288-evb

saradc in rk3288-evb use 1.8v ref.
Signed-off-by: Jacob Chen <jacob-chen@iotwrt.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm/boot/dts/rk3288-evb.dtsi

@@ -218,6 +218,11 @@ &emmc {
 	status = "okay";
 };
 
+&saradc {
+	vref-supply = <&vcc_18>;
+	status = "okay";
+};
+
 &sdmmc {
 	bus-width = <4>;
 	cap-mmc-highspeed;