dt-bindings: hwmon: Add IIO HWMON binding

Even though we had the iio-hwmon driver for some time and a number of
boards using it already, we never had a binding for it. Let's add it
based on what the driver expects and the boards are providing.

Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Jean Delvare <jdelvare@suse.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-hwmon@vger.kernel.org
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210901091852.479202-24-maxime@cerno.techSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/iio-hwmon.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/hwmon/iio-hwmon.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: ADC-attached Hardware Sensor Device Tree Bindings
+
+maintainers:
+  - Jonathan Cameron <jic23@kernel.org>
+
+description: >
+  Bindings for hardware monitoring devices connected to ADC controllers
+  supporting the Industrial I/O bindings.
+
+properties:
+  compatible:
+    const: iio-hwmon
+
+  io-channels:
+    minItems: 1
+    maxItems: 8 # Should be enough
+    description: >
+      List of phandles to ADC channels to read the monitoring values
+
+required:
+  - compatible
+  - io-channels
+
+additionalProperties: false
+
+examples:
+  - |
+      iio-hwmon {
+          compatible = "iio-hwmon";
+          io-channels = <&adc 1>, <&adc 2>;
+      };