iio: humidity: Add support for AM2315

Add basic support for the Aosong AM2315 relative humidity
and ambient temperature sensor.
Includes support for raw readings and ACPI detection.

Datasheet:
http://www.aosong.com/asp_bin/Products/en/AM2315.pdf



Signed-off-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>