dt-bindings:iio:dac:adi,ad7303 yaml conversion

Converted to maintain the requirement for Vdd-supply as per original file.
It is possible we could relax this requirement to make it at least one
of Vdd-supply and REF-supply.  We need to establish the scaling of the
output channel and if REF-supply is provided that is used instead of
Vdd-supply, hence I cannot see why a dummy regulator cannot be used for
Vdd-supply if this happens.

For now, let us keep it simple.

Drop adi,use-external-reference from binding example as no such binding
exists.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20201031134110.724233-18-jic23@kernel.org

Documentation/devicetree/bindings/iio/dac/ad7303.txt

-Analog Devices AD7303 DAC device driver
-
-Required properties:
-	- compatible: Must be "adi,ad7303"
-	- reg: SPI chip select number for the device
-	- spi-max-frequency: Max SPI frequency to use (< 30000000)
-	- Vdd-supply: Phandle to the Vdd power supply
-
-Optional properties:
-	- REF-supply: Phandle to the external reference voltage supply. This should
-	  only be set if there is an external reference voltage connected to the REF
-	  pin. If the property is not set Vdd/2 is used as the reference voltage.
-
-Example:
-
-		ad7303@4 {
-			compatible = "adi,ad7303";
-			reg = <4>;
-			spi-max-frequency = <10000000>;
-			Vdd-supply = <&vdd_supply>;
-			adi,use-external-reference;
-			REF-supply = <&vref_supply>;
-		};

Documentation/devicetree/bindings/iio/dac/adi,ad7303.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/dac/adi,ad7303.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Analog Devices AD7303 DAC
+
+maintainers:
+  - Lars-Peter Clausen <lars@metafoo.de>
+
+properties:
+  compatible:
+    const: adi,ad7303
+
+  reg:
+    maxItems: 1
+
+  Vdd-supply:
+    description:
+      Used to calculate output channel scalling if REF-supply not specified.
+  REF-supply:
+    description:
+      If not provided, Vdd/2 is used as the reference voltage.
+
+  spi-max-frequency:
+    maximum: 30000000
+
+required:
+  - compatible
+  - reg
+  - Vdd-supply
+
+additionalProperties: false
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        dac@4 {
+            compatible = "adi,ad7303";
+            reg = <4>;
+            spi-max-frequency = <10000000>;
+            Vdd-supply = <&vdd_supply>;
+            REF-supply = <&vref_supply>;
+        };
+    };
+...