dt-bindings: iio: frequency: Add docs for ADF4371 PLL

Document support for Analog Devices ADF4371 SPI Wideband Synthesizer.
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/frequency/adf4371.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/frequency/adf4371.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Analog Devices ADF4371 Wideband Synthesizer
+
+maintainers:
+  - Popa Stefan <stefan.popa@analog.com>
+
+description: |
+  Analog Devices ADF4371 SPI Wideband Synthesizer
+  https://www.analog.com/media/en/technical-documentation/data-sheets/adf4371.pdf
+
+properties:
+  compatible:
+    enum:
+      - adi,adf4371
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    description:
+      Definition of the external clock (see clock/clock-bindings.txt)
+    maxItems: 1
+
+  clock-names:
+    description:
+      Must be "clkin"
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - clock-names
+
+examples:
+  - |
+    spi0 {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        frequency@0 {
+                compatible = "adi,adf4371";
+                reg = <0>;
+                spi-max-frequency = <1000000>;
+                clocks = <&adf4371_clkin>;
+                clock-names = "clkin";
+        };
+    };
+...