dt-bindings: iio: adc: ad7291: add binding

Add device-tree binding for ADI AD7291 ADC.
Signed-off-by: Michael Auchter <michael.auchter@ni.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200922144422.542669-2-michael.auchter@ni.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/adi,ad7291.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/adc/adi,ad7291.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: AD7291 8-Channel, I2C, 12-Bit SAR ADC with Temperature Sensor
+
+maintainers:
+  - Michael Auchter <michael.auchter@ni.com>
+
+description: |
+  Analog Devices AD7291 8-Channel I2C 12-Bit SAR ADC with Temperature Sensor
+  https://www.analog.com/media/en/technical-documentation/data-sheets/ad7291.pdf
+
+properties:
+  compatible:
+    enum:
+      - adi,ad7291
+
+  reg:
+    maxItems: 1
+
+  vref-supply:
+    description: |
+      The regulator supply for ADC reference voltage.
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    i2c {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      ad7291: adc@0 {
+        compatible = "adi,ad7291";
+        reg = <0>;
+        vref-supply = <&adc_vref>;
+      };
+    };
+...
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