ARM: S3C64XX: Fix build for implicit serial_s3c.h inclusion

Some very recent change appears to have removed an implicit inclusion of
serial_s3c.h causing build failures due to references to UART registers
in the serial port restore code in next-20140318.  Include it explicitly
to fix the build.

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>