Analog Devices AD7303 DAC device driver

Required properties:
	- compatible: Must be "adi,ad7303"
	- reg: SPI chip select number for the device
	- spi-max-frequency: Max SPI frequency to use (< 30000000)
	- Vdd-supply: Phandle to the Vdd power supply

Optional properties:
	- REF-supply: Phandle to the external reference voltage supply. This should
	  only be set if there is an external reference voltage connected to the REF
	  pin. If the property is not set Vdd/2 is used as the reference voltage.

Example:

		ad7303@4 {
			compatible = "adi,ad7303";
			reg = <4>;
			spi-max-frequency = <10000000>;
			Vdd-supply = <&vdd_supply>;
			adi,use-external-reference;
			REF-supply = <&vref_supply>;
		};