dt-bindings:iio:temperature:maxim,max31856 yaml conversion.

Simple txt to yaml conversion of this binding.
Paresh Chaudhary's email is bouncing so for now I've listed
myself as maintainer.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031134110.724233-28-jic23@kernel.org