dt-bindings:iio:potentiometer:maxim,ds1803 move to trivial devices.

Simple binding where there is no obvious benefit in maintaining a
separate file.  Hence document in trivial-devices.yaml and drop
the txt file.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Slawomir Stepien <sst@poczta.fm>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Slawomir Stepien <sst@poczta.fm>
Link: https://lore.kernel.org/r/20201031182922.743153-4-jic23@kernel.org