dt-bindings:iio:potentiometer:microchip,mcp4131 txt to yaml conversion

This binding is very simple, but I think the very large number of
compatible values make it unsuitable for moving to trivial-devices.yaml.
Main change in the conversion was reordering the compatible list to
numerical order.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Slawomir Stepien <sst@poczta.fm>
Link: https://lore.kernel.org/r/20201031184854.745828-4-jic23@kernel.org