Documentation: dt: iio: add mcp4725/6 dac device binding

Signed-off-by: NTomas Novotny <tomas@novotny.cz>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/dac/mcp4725.txt

+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;
+	};