scripts/dts: Remove dead and confusing pwm/gpio-related code

 - Maybe the code is trying to turn integers into single-element list,
   but it won't work (needs to be [value] rather than list(value)).
   Don't think you could ever end up with an integer either.

 - extract_controller() assumes that prop_values is a list, so
   reduced[...].get(prop) should be reduced[...][prop]. get() means it's
   okay for the key to be missing, which makes the code confusing.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>