iio: core: add thermocouple_type standard attribute

We have a couple of thermocouple IIO drivers, supporting several chips.
Some of them support only one specific thermocouple type (e.g. "K", "J"),
one of them can be configured to work with several different thermocouple
types.

In certain applications thermocouples could be externally connected to the
chip by the user.

This patch introduces a new IIO standard attribute to report the supported
thermocouple type and, where applicable, to allow it to be dynamically set
using sysfs.

Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Patrick Havelange <patrick.havelange@essensium.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: Chuhong Yuan <hslester96@gmail.com>
Cc: Daniel Gomez <dagmcr@gmail.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>