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

Signed-off-by: Tomas Novotny <tomas@novotny.cz>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

diff --git a/Documentation/devicetree/bindings/iio/dac/mcp4725.txt b/Documentation/devicetree/bindings/iio/dac/mcp4725.txt
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+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;
+       };