iio: temperature: add support for Maxim thermocouple chips

Add initial driver support for MAX6675, and MAX31855 thermocouple chips.

Cc: Marek Vasut <marex@denx.de>
Cc: Matt Porter <mporter@konsulko.com>
Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>