dt-bindings: iio: frequency: Add ADMV4420 doc

Add device tree bindings for the ADMV4420 K band downconverter.
Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20220223130808.13352-1-cristian.pop@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/frequency/adi,admv4420.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/frequency/adi,admv4420.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ADMV4420 K Band Downconverter
+
+maintainers:
+  - Cristian Pop <cristian.pop@analog.com>
+
+description:
+  The ADMV4420 is a highly integrated, double balanced, active
+  mixer with an integrated fractional-N synthesizer, ideally suited
+  for next generation K band satellite communications
+
+properties:
+  compatible:
+    enum:
+      - adi,admv4420
+
+  reg:
+    maxItems: 1
+
+  spi-max-frequency:
+    maximum: 1000000
+
+  adi,lo-freq-khz:
+    description: LO Frequency
+    $ref: /schemas/types.yaml#/definitions/uint32
+
+  adi,ref-ext-single-ended-en:
+    description: External reference selected.
+    type: boolean
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    spi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      mixer@0 {
+        compatible = "adi,admv4420";
+        reg = <0>;
+        spi-max-frequency = <1000000>;
+        adi,lo-freq-khz = <16750000>;
+        adi,ref-ext-single-ended-en;
+      };
+    };
+...