ROCKCHIP with MAX98357A/RT5514/DA7219 codecs on GRU boards

Required properties:
- compatible: "rockchip,rk3399-gru-sound"
- rockchip,cpu: The phandle of the Rockchip I2S controller that's
  connected to the codecs
- rockchip,codec: The phandle of the MAX98357A/RT5514/DA7219 codecs

Example:

sound {
	compatible = "rockchip,rk3399-gru-sound";
	rockchip,cpu = <&i2s0>;
	rockchip,codec = <&max98357a &rt5514 &da7219>;
};