dt-bindings:iio:potentiometer:maxim,max5481 move to trivial devices

Simple SPI binding that doesn't need a separate file.
During conversion I looked up the individual part number descriptions
in the datasheet so that we could give slightly more detail in
trivial-device.yaml.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Slawomir Stepien <sst@poczta.fm>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Maury Anderson <maury.anderson@rockwellcollins.com>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Cc: Slawomir Stepien <sst@poczta.fm>
Link: https://lore.kernel.org/r/20201031182922.743153-5-jic23@kernel.org