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 <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/temperature/maxim_thermocouple.c

···
 static const struct spi_device_id maxim_thermocouple_id[] = {
 	{"max6675", MAX6675},
 	{"max31855", MAX31855},
+	{"max31856", MAX31855},
 	{},
 };
 MODULE_DEVICE_TABLE(spi, maxim_thermocouple_id);