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.tech


Signed-off-by: Guenter Roeck <linux@roeck-us.net>