ASoC: Add DT bindings documentation for max98504 amplifier

Maxim MAX98504 class D mono speaker amplifier

This device supports I2C control interface and an IRQ output signal. It features

a PCM and PDM digital audio interface (DAI) and a differential analog input.

Required properties:

 - compatible : "maxim,max98504"

 - reg : should contain the I2C slave device address

 - DVDD-supply, DIOVDD-supply, PVDD-supply: power supplies for the device,

   as covered in ../regulator/regulator.txt

 - interrupts : should specify the interrupt line the device is connected to,

   as described in ../interrupt-controller/interrupts.txt

Optional properties:

 - maxim,brownout-threshold - the PVDD brownout threshold, the value must be

   from 0, 1...21 range, corresponding to 2.6V, 2.65V...3.65V voltage range

 - maxim,brownout-attenuation - the brownout attenuation to the speaker gain

   applied during the "attack hold" and "timed hold" phase, the value must be

   from 0...6 (dB) range

 - maxim,brownout-attack-hold-ms - the brownout attack hold phase time in ms,

   0...255 (VBATBROWN_ATTK_HOLD, register 0x0018)

 - maxim,brownout-timed-hold-ms - the brownout timed hold phase time in ms,

   0...255 (VBATBROWN_TIME_HOLD, register 0x0019)

 - maxim,brownout-release-rate-ms - the brownout release phase step time in ms,

   0...255 (VBATBROWN_RELEASE, register 0x001A)

The default value when the above properties are not specified is 0,

the maxim,brownout-threshold property must be specified to actually enable

the PVDD brownout protection.

Example:

 max98504@31 {

	compatible = "maxim,max98504";

	reg = <0x31>;

	interrupt-parent = <&gpio_bank_0>;

	interrupts = <2 0>;

	DVDD-supply = <&regulator>;

	DIOVDD-supply = <&regulator>;

	PVDD-supply = <&regulator>;

};