[ARM] Ensure CONFIG_SERIAL_SAMSUNG_UARTS is always set.

Always set CONFIG_SERIAL_SAMSUNG_UARTS when building any
of the S3C platforms as even if the driver is not selected
there it is still the facility for the machine files to
register configuration data for the possibility of the
driver being built.

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