ARM: EXYNOS: Fix compilation error in cpuidle.c

The big series refactoring Exynos suspend to RAM handling missed the
cpuidle driver that is disabled in exynos_defconfig, leaving it
including old mach/pm_core.h header and using old s3c_cpu_resume symbol
instead of new exynos_cpu_resume, resulting in compilation failures with
CONFIG_ARCH_EXYNOS and CONFIG_CPU_IDLE enabled.

This patch fixes that silly mistake and performs necessary modification
to arhc/arm/exynos/cpuidle.c to make it compile again.

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