hwmon: (iio_hwmon) Add support for humidity sensors

The iio subsystem supports humidity sensors, so it makes sense
to support it in the iio-hwmon bridge as well.

Cc: Jonathan Cameron <jic23@kernel.org>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>