staging:iio:adc:lpc32xx Move out of staging.

There are a few more little cleanups that could be done on this driver, but
I don't think any are sufficient to justify not moving it out of staging.

It's a very simple driver (presumably for a simple part) so not much that can
go wrong.  I think it was only ever in staging because that's where IIO was
as a whole at the time and then we forgot about it!

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Roland Stigge <stigge@antcom.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>