dt-bindings: iio: amplifiers: add ada4250 doc

Add device tree bindings for the ADA4250 driver.
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220223120112.8067-1-antoniu.miclaus@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/amplifiers/adi,ada4250.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/amplifiers/adi,ada4250.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ADA4250 Programmable Gain Instrumentation Amplifier
+
+maintainers:
+  - Antoniu Miclaus <antoniu.miclaus@analog.com>
+
+description: |
+  Precision Low Power, 110kHz, 26uA, Programmable Gain Instrumentation Amplifier.
+
+properties:
+  compatible:
+    enum:
+      - adi,ada4250
+
+  reg:
+    maxItems: 1
+
+  avdd-supply: true
+
+  adi,refbuf-enable:
+    description:
+      Enable internal buffer to drive the reference pin.
+    type: boolean
+
+  spi-max-frequency: true
+
+required:
+  - compatible
+  - reg
+  - avdd-supply
+
+additionalProperties: false
+
+examples:
+  - |
+    spi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      amplifier@0 {
+        compatible = "adi,ada4250";
+        reg = <0>;
+        avdd-supply = <&avdd>;
+      };
+    };
+...