ROCKCHIP Built-in HDMI and DP audio

Required properties:
- compatible: "rockchip,rk3399-hdmi-dp"
- rockchip,cpu: The phandle of the Rockchip I2S controller that's
  connected to the CODEC
- rockchip,codec: The phandle of audio codecs, should contain HDMI
  or DP, or both.

Example:

sound {
	compatible = "rockchip,rk3399-hdmi-dp";
	rockchip,cpu = <&i2s2>;
	rockchip,codec = <&hdmi>, <&cdn_dp>;
};