Microchip mcp4725 and mcp4726 DAC device driver

Required properties:
	- compatible: Must be "microchip,mcp4725" or "microchip,mcp4726"
	- reg: Should contain the DAC I2C address
	- vdd-supply: Phandle to the Vdd power supply. This supply is used as a
	  voltage reference on mcp4725. It is used as a voltage reference on
	  mcp4726 if there is no vref-supply specified.

Optional properties (valid only for mcp4726):
	- vref-supply: Optional phandle to the Vref power supply. Vref pin is
	  used as a voltage reference when this supply is specified.
	- microchip,vref-buffered: Boolean to enable buffering of the external
	  Vref pin. This boolean is not valid without the vref-supply. Quoting
	  the datasheet: This is offered in cases where the reference voltage
	  does not have the current capability not to drop its voltage when
	  connected to the internal resistor ladder circuit.

Examples:

	/* simple mcp4725 */
	mcp4725@60 {
		compatible = "microchip,mcp4725";
		reg = <0x60>;
		vdd-supply = <&vdac_vdd>;
	};

	/* mcp4726 with the buffered external reference voltage */
	mcp4726@60 {
		compatible = "microchip,mcp4726";
		reg = <0x60>;
		vdd-supply = <&vdac_vdd>;
		vref-supply = <&vdac_vref>;
		microchip,vref-buffered;
	};