arm64: dts: rockchip: enable hdmi audio out for rk3399-rockpro64

The rockpro64 has hdmi support. So this patch enables hdmi audio
feature that is defined in rk3399 devicetree.
Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399-rockpro64.dts

@@ -222,6 +222,10 @@ &hdmi {
 	status = "okay";
 };
 
+&hdmi_sound {
+	status = "okay";
+};
+
 &gpu {
 	mali-supply = <&vdd_gpu>;
 	status = "okay";