ARM: S3C64XX: Fix missing header error with CONFIG_CPU_IDLE enabled

Recently the regs-syscon-power.h header was moved from
mach-s3c64xx/include/mach to mach-s3c64xx/, but cpuidle.c was not
updated to include the header from its new location, which caused
build error with CONFIG_CPU_IDLE enabled.

This patch fixed the problem by updating the include line with
proper header location.

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