iio: Add si7005 relative humidity and temperature sensor driver

sensor provides 12-bit relative humidity and 14-bit temperature
via I2C interface; temperature and linearity compensation is not
implemented (yet)

driver also supports the Si7015, but not the 2nd generation
sensors Si7013/Si7020/Si7021

datasheet is here
http://www.silabs.com/Support%20Documents/TechnicalDocs/Si7005.pdf



v2: (thanks to Lars-Peter Clausen)
* fix coding style
* use devm_iio_device_register()
* change copyright year to 2014

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>