serial: s3c: Fix build of header without serial_core.h preinclusion

serial_s3c.h uses upf_t which is defined in serial_core.h but does not
include that itself meaning that users which include serial_s3c.h by
itself don't build.

Signed-off-by: Mark Brown <broonie@linaro.org>
[t.figa: Moved inclusion under #ifndef __ASSEMBLY__ to fix mach-exynos]
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>