iio: temperature: maxim_thermocouple: add MAX31856 part

MAX31856 is register equivalent to the MAX31855 but suppports J, N, R,
S, T, E and B type thermocouples in addition to K-type.

Data conversion for the various types happens transparently to the driver
via probe type detection, and a LUT on the sensor.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>