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>