[ARM] S3C: Update CONFIG_SERIAL_SAMSUNG_UARTS handling

Fix the usage of CONFIG_SERIAL_SAMSUNG_UARTS in several places
in the kernel where it had been missed. This finishes fixing a
long standing issue where S3C2443 and S3C64XX could not use the
4th UART

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