iio: Add Melexis mlx90614 contact-less infrared temperature sensor driver

I2C-controlled sensor measures ambient and object temperatuer

see
http://www.melexis.com/Infrared-Thermometer-Sensors/Infrared-Thermometer-Sensors/MLX90614-615.aspx



Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>