dt-bindings: iio/temperature: Add thermocouple types (and doc)

This patch introduces common thermocouple types used by various
temperature sensors. Also a brief documentation explaining this
"thermocouple-type" property.

Signed-off-by: Patrick Havelange <patrick.havelange@essensium.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>