dt-bindings: iio: dac: Maxim max5522 DAC

Add binding Doc for this SPI DAC.

The driver was perviously posted but was missing the DT binding document.
https://lore.kernel.org/all/20221106165928.223318-1-angelo.dureghello@timesys.com/Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: NAngelo Dureghello <angelo.dureghello@timesys.com>
Reviewed-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230115173958.134201-1-jic23@kernel.org

Documentation/devicetree/bindings/iio/dac/maxim,max5522.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/dac/maxim,max5522.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Maxim Integrated MAX5522 Dual 10-bit Voltage-Output SPI DACs
+
+maintainers:
+  - Angelo Dureghello <angelo.dureghello@timesys.com>
+  - Jonathan Cameron <jic23@kernel.org>
+
+description: |
+  Datasheet available at:
+  https://www.analog.com/en/products/max5522.html
+
+properties:
+  compatible:
+    const: maxim,max5522
+
+  reg:
+    maxItems: 1
+
+  vdd-supply: true
+  vrefin-supply: true
+
+required:
+  - compatible
+  - reg
+  - vrefin-supply
+
+allOf:
+  - $ref: /schemas/spi/spi-peripheral-props.yaml#
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        dac@0 {
+            compatible = "maxim,max5522";
+            reg = <0>;
+            vrefin-supply = <&vref>;
+        };
+    };
+...