dt-bindings: iio: dac: ti-dac082s085: Document new driver

Texas Instruments 8/10/12-bit 2/4-channel DAC driver

Required properties:

 - compatible:		Must be one of:

			"ti,dac082s085"

			"ti,dac102s085"

			"ti,dac122s085"

			"ti,dac084s085"

			"ti,dac104s085"

			"ti,dac124s085"

 - reg: 		Chip select number.

 - spi-cpha, spi-cpol:	SPI mode (0,1) or (1,0) must be used, so specify

			either spi-cpha or spi-cpol (but not both).

 - vref-supply: 	Phandle to the external reference voltage supply.

For other required and optional properties of SPI slave nodes please refer to

../../spi/spi-bus.txt.

Example:

	vref_2v5_reg: regulator-vref {

		compatible = "regulator-fixed";

		regulator-name = "2v5";

		regulator-min-microvolt = <2500000>;

		regulator-max-microvolt = <2500000>;

		regulator-always-on;

	};

	dac@0 {

		compatible = "ti,dac082s085";

		reg = <0>;

		spi-max-frequency = <40000000>;

		spi-cpol;

		vref-supply = <&vref_2v5_reg>;

	};