TI DAC7311 device tree bindings

Required properties:
- compatible: must be set to:
	* "ti,dac7311"
	* "ti,dac6311"
	* "ti,dac5311"
- reg: spi chip select number for the device
- vref-supply: The regulator supply for ADC reference voltage

Optional properties:
- spi-max-frequency: Max SPI frequency to use

Example:

	spi_master {
		dac@0 {
			compatible = "ti,dac7311";
			reg = <0>; /* CS0 */
			spi-max-frequency = <1000000>;
			vref-supply = <&vdd_supply>;
		};
	};