Add support for more chips to the gpioths driver.

Previously the driver supported the DHT11 sensor. Now it supports

DHT11, DHT12, DHT21, DHT22, AM3201, AM3202.

All these chips are similar, differing primarily in supported temperature
and humidity ranges and accuracy (and, presumably, cost). There are two
basic data formats reported by the various chips, and it is possible to
figure out at runtime which format to use for decoding the data based on
the range of values in a single byte of the humidity measurement. (which
is detailed in a comment block, so I won't recapitulate it here).