dt-bindings:iio:temperature:maxim_thermocouple.txt to maxim,max31855k.yaml

Given we already have another maxim thermocouple driver that isn't
covered by this binding it seems a better idea to chose to name it
after a specific part.

I added an additional example for the maxim,max6755 to illustrate
the need for spi-cpha for that part.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20201031134110.724233-29-jic23@kernel.org