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: NMatt Ranostay <mranostay@gmail.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>