dt-bindings: iio: dac: ad5686 and ad5696: Add missing binding document.

The use of vcc-supply for the reference voltage is unusual and should
probably be deprecated as there is an explicit VREF pin on at least
some of these parts.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-11-jic23@kernel.org

Documentation/devicetree/bindings/iio/dac/adi,ad5686.yaml

@@ -0,0 +1,75 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/dac/adi,ad5686.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Analog Devices AD5360 and similar DACs
+
+maintainers:
+  - Michael Hennerich <michael.hennerich@analog.com>
+  - Jonathan Cameron <jic23@kernel.org>
+
+properties:
+  compatible:
+    oneOf:
+      - description: SPI devices
+        enum:
+          - adi,ad5310r
+          - adi,ad5672r
+          - adi,ad5674r
+          - adi,ad5676
+          - adi,ad5676r
+          - adi,ad5679r
+          - adi,ad5681r
+          - adi,ad5682r
+          - adi,ad5683
+          - adi,ad5683r
+          - adi,ad5684
+          - adi,ad5684r
+          - adi,ad5685r
+          - adi,ad5686
+          - adi,ad5686r
+      - description: I2C devices
+        enum:
+          - adi,ad5311r
+          - adi,ad5338r
+          - adi,ad5671r
+          - adi,ad5675r
+          - adi,ad5691r
+          - adi,ad5692r
+          - adi,ad5693
+          - adi,ad5693r
+          - adi,ad5694
+          - adi,ad5694r
+          - adi,ad5695r
+          - adi,ad5696
+          - adi,ad5696r
+
+
+  reg:
+    maxItems: 1
+
+  vcc-supply:
+    description: If not supplied the internal reference is used.
+
+  spi-max-frequency: true
+
+additionalProperties: false
+
+required:
+  - compatible
+  - reg
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+        dac@0 {
+            reg = <0>;
+            compatible = "adi,ad5310r";
+            vcc-supply = <&dac_vref0>;
+        };
+    };
+...