Add support for DHT-22

[Dzeri96]

I'm poking around the code and I'd like some guidance on how to make a PR that enables DHT-22 support.
After enabling the overlay in the boot settings, I'm able to read the raw values via IIO_READER in python like so:

from reading import IIO_READER
import iio

sensor = IIO_READER()
sensor.get_readings(iio.Context())

However, idetect is only scanning for i2c devices and the DHT-22 is not initialized. What do I have to do to fix this?

If I skip the check for the device count in sensors.py, everything seems to work fine, except the raw values from the sensor are shown. We need to convert these to actual temp/humidity units.