ARM: S5P: 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-s5p6440/cpu.c
arch/arm/mach-s5p6442/cpu.c
arch/arm/mach-s5pc100/cpu.c
arch/arm/mach-s5pv210/cpu.c

Following is error message of in case of s5pv210_defconfig with CONFIG_PREEMPT_NONE.

arch/arm/mach-s5pv210/cpu.c:91: error: implicit declaration of function 'need_resched'

Signed-off-by: SeungChull Suh <sc.suh@samsung.com>
[kgene.kim@samsung.com: removed mach-s5p64x0/cpu.c]
[kgene.kim@samsung.com: added fix mach-s5p6440/cpu.c]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>