dt-bindings:iio:adc:adi,ad7298 document bindings

The device has a tsens-busy pin, but it's both fiddly and currently
ignored by the Linux driver.  Given it's not clear whether the binding
should be an interrupt, or a GPIO I have left that out for now.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210401174112.320497-7-jic23@kernel.org

Documentation/devicetree/bindings/iio/adc/adi,ad7298.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright 2019 Analog Devices Inc.
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/adc/adi,ad7298.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Analog Devices AD7298 ADC
+
+maintainers:
+  - Michael Hennerich <michael.hennerich@analog.com>
+
+description: |
+  Bindings for the Analog Devices AD7298 ADC device. Datasheet can be
+  found here:
+    https://www.analog.com/en/products/ad7298.html
+
+properties:
+  compatible:
+    const: adi,ad7298
+
+  reg:
+    maxItems: 1
+
+  vref-supply: true
+  vdd-supply: true
+  spi-max-frequency: true
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    spi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      adc@0 {
+        compatible = "adi,ad7298";
+        reg = <0>;
+        spi-max-frequency = <5000000>;
+        vref-supply = <&adc_vref>;
+      };
+    };
+...