Linear Technology LTC2632 DAC device driver

Required properties:
 - compatible: Has to contain one of the following:
	lltc,ltc2632-l12
	lltc,ltc2632-l10
	lltc,ltc2632-l8
	lltc,ltc2632-h12
	lltc,ltc2632-h10
	lltc,ltc2632-h8

Property rules described in Documentation/devicetree/bindings/spi/spi-bus.txt
apply. In particular, "reg" and "spi-max-frequency" properties must be given.

Optional properties:
	- vref-supply: Phandle to the external reference voltage supply. This should
	  only be set if there is an external reference voltage connected to the VREF
	  pin. If the property is not set the internal reference is used.

Example:

	vref: regulator-vref {
		compatible = "regulator-fixed";
		regulator-name = "vref-ltc2632";
		regulator-min-microvolt = <1250000>;
		regulator-max-microvolt = <1250000>;
		regulator-always-on;
	};

	spi_master {
		dac: ltc2632@0 {
			compatible = "lltc,ltc2632-l12";
			reg = <0>; /* CS0 */
			spi-max-frequency = <1000000>;
			vref-supply = <&vref>; /* optional */
		};
	};