Texas Instruments TAS5760 General purpose class-D amplifier.

The codec is controlled through and I2C interface. It also has an optional
speaker shutdown(SPK_SD) signal which can be wired to a GPIO.

Required properties:

- compatible: "ti, tas5760"
- reg : The I2C address of the device
- #sound-dai-cells: must be equal to 0

Optional properties:

- spk-sd-gpios: GPIO specifier for TAS5760's active low speaker shutdown line
- hw_params: subnode to specify the config parameters used for initialisation
  of chip. It consists of the following optional properties:
  	- pbtl-mode: selection for PBTL/BTL mode
	- pbtl-right-channel: channel selection for PBTL mode
	- volume-fading: enable/disable volume fading
	- digital-clip: value for adjusting digital clip level
	  range: (0 to 0xFFFFF)
	- digital-boost: digital gain of the amplifier
	  possible values(dB): 0,6,12,18
	- analog-gain: value for adjusting analog gain of the amplifier
	  possible values(0.1 dBV): 192, 226, 250
	- pwm-rate: control for pwm output swtching rate
	  range: (0 to 7)

Example:
	tas5760@06c {
		compatible: "ti,tas5760";
		reg = <0x6c>;
		#sound-dai-cells = <0>;

		spk-sd-gpios = <&gpio1 5 GPIO_ACTIVE_LOW>;

		hw_params {
			pbtl-mode;
			pbtl-right-channel;
			digital-clip = <0XFFFFF>;
			volume-fading;
			digital-boost = <6>;
			analog-gain = <192>;
			pwm-rate = <5>;
		};
	};