dt-bindings:iio:filter: add admv8818 doc

Add device tree bindings for the ADMV8818 Filter.
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/filter/adi,admv8818.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/filter/adi,admv8818.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ADMV8818 Digitally Tunable, High-Pass and Low-Pass Filter
+
+maintainers:
+  - Antoniu Miclaus <antoniu.miclaus@analog.com>
+
+description: |
+    Fully monolithic microwave integrated circuit (MMIC) that
+    features a digitally selectable frequency of operation.
+    The device features four independently controlled high-pass
+    filters (HPFs) and four independently controlled low-pass filters
+    (LPFs) that span the 2 GHz to 18 GHz frequency range.
+
+    https://www.analog.com/en/products/admv8818.html
+
+properties:
+  compatible:
+    enum:
+      - adi,admv8818
+
+  reg:
+    maxItems: 1
+
+  spi-max-frequency:
+    maximum: 10000000
+
+  clocks:
+    description:
+      Definition of the external clock.
+    minItems: 1
+
+  clock-names:
+    items:
+      - const: rf_in
+
+  clock-output-names:
+    maxItems: 1
+
+  '#clock-cells':
+    const: 0
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    spi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      admv8818@0 {
+        compatible = "adi,admv8818";
+        reg = <0>;
+        spi-max-frequency = <10000000>;
+        clocks = <&admv8818_rfin>;
+        clock-names = "rf_in";
+      };
+    };
+...