dt-bindings: iio: dac: adi,ad8801: Add missing binding document.

This device was missing a binding document. Simple binding with the
ad8803 requiring two both low and high references whilst the 8801
only has a high reference.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-15-jic23@kernel.org

Documentation/devicetree/bindings/iio/dac/adi,ad8801.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/dac/adi,ad8801.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Analog Devices AD8801 and AD8803 DACs
+
+maintainers:
+  - Jonathan Cameron <jic23@kernel.org>
+
+properties:
+
+  compatible:
+    enum:
+      - adi,ad8801
+      - adi,ad8803
+
+  reg:
+    maxItems: 1
+
+  spi-max-frequency: true
+
+  vrefh-supply: true
+  vrefl-supply: true
+
+additionalProperties: false
+
+required:
+  - compatible
+  - reg
+  - vrefh-supply
+
+allOf:
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: adi,ad8803
+    then:
+      required:
+        - vrefl-supply
+    else:
+      properties:
+        vrefl-supply: false
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        dac@0 {
+            compatible = "adi,ad8803";
+            reg = <0>;
+            vrefl-supply = <&dac_vrefl>;
+            vrefh-supply = <&dac_vrefh>;
+        };
+    };
+...