ARM: S5P64X0: Explicitly include linux/serial_s3c.h in mach/pm-core.h

This patch fixes compilation failure due to missing explicit inclusion
of linux/serial_s3c.h in mach/pm-core.h, which contains definitions
required for further code in this header.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>