sensors: add driver for DHTxx sensors

Add driver for the DHT temperature and relative himidity sensor family.
The sensor has a single signal pin (in addition to GND and VDD) over
which communication is done in a half-duplex manner (see datasheet for
more details).

Datasheets:
	http://www.micropik.com/PDF/dht11.pdf
	https://www.sparkfun.com/datasheets/Sensors/Temperature/DHT22.pdf
	http://akizukidenshi.com/download/ds/aosong/AM2302.pdf



Origin: Original
Change-Id: If8c8be73d7fdfef2ab2091bf9b8fe72bcd700ed1
Signed-off-by: Bogdan Davidoaia <bogdan.m.davidoaia@intel.com>