iio: humidity: hdc3020: add threshold events support

Add threshold events support for temperature and relative humidity. To
enable them the higher and lower threshold registers must be programmed
and the higher threshold must be greater then or equal to the lower
threshold. Otherwise the event is disabled. Invalid hysteresis values
are ignored by the device. There is no further configuration possible.

Tested by setting thresholds/hysteresis and turning the heater on/off.
Used iio_event_monitor in tools/iio to catch events while constantly
displaying temperature and humidity values.

Signed-off-by: Dimitri Fedrau <dima.fedrau@gmail.com>
Tested-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://lore.kernel.org/r/20240214085350.19382-4-dima.fedrau@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>