[ARM] S3C24XX: Split serial driver into core and per-cpu drivers

The S3C2410 serial driver in drivers/serial/s3c2410.c has been
growing bigger with the addition of more variants of this hardware
with the growing Samsung SoCs range. As such, it would be
easier to split this code up into a core and per-cpu drivers to
make driver addition easier, and the core smaller.

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