dt-bindings: iio: maxim_thermocouple: document new 'compatible' strings

dt-bindings: iio: maxim_thermocouple: document new 'compatible' strings

Now the maxim_thermocouple has new, more specific, 'compatible' strings for
better distinguish the various supported chips.

This patch updates the DT bindings documentation accordingly

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
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>