dt-bindings: iio: dac: adi,ad5764: Add missing binding document

This binding covers the ad5744, ad5744r, ad5764 and ad5764r DACs.
Note that the driver currently assumes the internal reference is used for
the r parts. The binding as defined relaxes this constraint.

There is no support in the binding or driver for the two digital IO
signals.  I do not propose to add that until we have a means to
test any such binding.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-13-jic23@kernel.org

Documentation/devicetree/bindings/iio/dac/adi,ad5764.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/dac/adi,ad5764.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Analog Devices AD5744 and AD5764 DAC families
+
+maintainers:
+  - Lars-Peter Clausen <lars@metafoo.de>
+  - Jonathan Cameron <jic23@kernel.org>
+
+properties:
+
+  compatible:
+    enum:
+      - adi,ad5744
+      - adi,ad5744r
+      - adi,ad5764
+      - adi,ad5764r
+
+  reg:
+    maxItems: 1
+
+  spi-max-frequency: true
+
+  vrefAB-supply: true
+  vrefCD-supply: true
+
+additionalProperties: false
+
+required:
+  - compatible
+  - reg
+
+allOf:
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - adi,ad5744
+              - adi,ad5764
+    then:
+      required:
+        - vrefAB-supply
+        - vrefCD-supply
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        dac@0 {
+            compatible = "adi,ad5744";
+            reg = <0>;
+            vrefAB-supply = <&dac_vref>;
+            vrefCD-supply = <&dac_vref>;
+        };
+    };
+...