dt-bindings: iio: dac: adi,ad5360: Add missing binding document

Bindings for the family of many channel DACs.  Fairly straight forward
with just a differing number of voltage references (an extra one
for the 40 channel ad4371)
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/20210627163244.1090296-4-jic23@kernel.org

Documentation/devicetree/bindings/iio/dac/adi,ad5360.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/dac/adi,ad5360.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Analog Devices AD5360 and similar DACs
+
+maintainers:
+  - Lars-Peter Clausen <lars@metafoo.de>
+  - Jonathan Cameron <jic23@kernel.org>
+
+properties:
+  compatible:
+    enum:
+      - adi,ad5360
+      - adi,ad5361
+      - adi,ad5363
+      - adi,ad5370
+      - adi,ad5371
+      - adi,ad5372
+      - adi,ad5373
+
+  reg:
+    maxItems: 1
+
+  vref0-supply: true
+  vref1-supply: true
+  vref2-supply: true
+
+  spi-max-frequency: true
+
+additionalProperties: false
+
+required:
+  - compatible
+  - reg
+  - vref0-supply
+  - vref1-supply
+
+allOf:
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - adi,ad5360
+              - adi,ad5361
+              - adi,ad5363
+              - adi,ad5370
+              - adi,ad5372
+              - adi,ad5373
+    then:
+      properties:
+        vref2-supply: false
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - adi,ad5371
+      then:
+        required:
+          - vref2-supply
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+        dac@0 {
+            reg = <0>;
+            compatible = "adi,ad5371";
+            vref0-supply = <&dac_vref0>;
+            vref1-supply = <&dac_vref1>;
+            vref2-supply = <&dac_vref2>;
+        };
+    };
+...