ARM: S5P64X0: Bug fix on errors of build with CONFIG_PREEMPT_NONE

This patch adds header <linux/sched.h> into the below files for build with
CONFIG_PREEMPT_NONE.

arch/arm/mach-s5p64x0/cpu.c

Signed-off-by: Seung-Chull Suh <sc.suh@samsung.com>
[kgene.kim@samsung.com: edited title and message]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>