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

There are some unanswered questions due to disagreements between the code
and various datasheets (including between different datasheets for the same
part).

I don't think that is necessarily a reason to keep it in staging however.
I'm partly posting this patch inorder to reignite debate and with a bit
of luck find someone who has one of these to test!

Signed-off-by: Jonathan Cameron <jic23@kernel.org>