dt-bindings: iio: dac: ad5761: Add missing binding doc.

Straight forward binding with vref optional for those part with internal
regulators, but required for those without.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Ricardo Ribalda <ribalda@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-12-jic23@kernel.org

Documentation/devicetree/bindings/iio/dac/adi,ad5761.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/dac/adi,ad5761.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Analog Devices AD5761 and similar DACs
+
+maintainers:
+  - Ricardo Ribalda <ribalda@kernel.org>
+  - Jonathan Cameron <jic23@kernel.org>
+
+properties:
+
+  compatible:
+    enum:
+      - adi,ad5721
+      - adi,ad5721r
+      - adi,ad5761
+      - adi,ad5761r
+
+  reg:
+    maxItems: 1
+
+  spi-max-frequency: true
+
+  vref-supply:
+    description: If not supplied, internal reference will be used.
+
+additionalProperties: false
+
+required:
+  - compatible
+  - reg
+
+allOf:
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - adi,ad5721
+              - adi,ad5761
+    then:
+      required:
+        - vref-supply
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        dac@0 {
+            compatible = "adi,ad5721";
+            reg = <0>;
+            vref-supply = <&dac_vref>;
+        };
+    };
+...