Bindings for DAC emulation using a digital potentiometer

It is assumed that the dpot is used as a voltage divider between the
current dpot wiper setting and the maximum resistance of the dpot. The
divided voltage is provided by a vref regulator.

                  .------.
   .-----------.  |      |
   | vref      |--'    .---.
   | regulator |--.    |   |
   '-----------'  |    | d |
                  |    | p |
                  |    | o |  wiper
                  |    | t |<---------+
                  |    |   |
                  |    '---'       dac output voltage
                  |      |
                  '------+------------+

Required properties:
- compatible: Should be "dpot-dac"
- vref-supply: The regulator supplying the voltage divider.
- io-channels: Channel node of the dpot to be used for the voltage division.
- io-channel-names: Should be "dpot".

Example:

	&i2c {
		dpot: mcp4651-503@28 {
			compatible = "microchip,mcp4651-503";
			reg = <0x28>;
			#io-channel-cells = <1>;
		};
	};

	dac {
		compatible = "dpot-dac";
		vref-supply = <&reg_3v3>;
		io-channels = <&dpot 0>;
		io-channel-names = "dpot";
	};