dt-bindings: staging: iio: cdc: ad7746: add binding documentation for AD7746

Add device tree binding documentation for AD7746 cdc in YAML format.
Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/2c2f4d486a94e0740e112bfac0d9306bdb7ea69c.1619841953.git.lucas.p.stankus@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/cdc/adi,ad7746.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/cdc/adi,ad7746.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: AD7746 24-Bit Capacitance-to-Digital Converter with Temperature Sensor
+
+maintainers:
+  - Michael Hennerich <michael.hennerich@analog.com>
+
+description: |
+  AD7746 24-Bit Capacitance-to-Digital Converter with Temperature Sensor
+
+  Specifications about the part can be found at:
+  https://www.analog.com/media/en/technical-documentation/data-sheets/ad7291.pdf
+
+properties:
+  compatible:
+    enum:
+      - adi,ad7745
+      - adi,ad7746
+      - adi,ad7747
+
+  reg:
+    maxItems: 1
+
+  adi,excitation-vdd-permille:
+    description: |
+      Set VDD per mille to be used as the excitation voltage.
+    $ref: /schemas/types.yaml#/definitions/uint32
+    enum: [125, 250, 375, 500]
+
+  adi,exca-output-en:
+    description: Enables the EXCA pin as the excitation output.
+    type: boolean
+
+  adi,exca-output-invert:
+    description: |
+      Inverts the excitation output in the EXCA pin.
+      Normally only one of the EXCX pins would be inverted, check the following
+      application notes for more details
+      https://www.analog.com/media/en/technical-documentation/application-notes/AN-1585.pdf
+    type: boolean
+
+  adi,excb-output-en:
+    description: Enables the EXCB pin as the excitation output.
+    type: boolean
+
+  adi,excb-output-invert:
+    description: Inverts the excitation output in the EXCB pin.
+    type: boolean
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    i2c {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      ad7746: cdc@48 {
+        compatible = "adi,ad7746";
+        reg = <0x48>;
+        adi,excitation-vdd-permille = <125>;
+
+        adi,exca-output-en;
+        adi,exca-output-invert;
+        adi,excb-output-en;
+        adi,excb-output-invert;
+      };
+    };
+...