iio: temperature: Add support for LTC2983

The LTC2983 is a Multi-Sensor High Accuracy Digital Temperature
Measurement System. It measures a wide variety of temperature sensors and
digitally outputs the result, in °C or °F, with 0.1°C accuracy and
0.001°C resolution. It can measure the temperature of all standard
thermocouples (type B,E,J,K,N,S,R,T), standard 2-,3-,4-wire RTDs,
thermistors and diodes.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>