ARM: S3C: Add ADC synchronous read call.

To add HWMON support, we need a synchronous read() call that blocks
until completion. Add the client that is being service to the select
and convert callbacks to make the code easier.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>