iio:temperature: Add MAX31856 thermocouple support

This patch adds support for Maxim MAX31856 thermocouple
temperature sensor support.

More information can be found in:
https://www.maximintegrated.com/en/ds/MAX31856.pdf



NOTE: Driver support only Comparator Mode.

Signed-off-by: Paresh Chaudhary <paresh.chaudhary@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Patrick Havelange <patrick.havelange@essensium.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>