ARM: EXYNOS: Adapt to cpuidle core time keeping and irq enable

This patch enables core cpuidle timekeeping and irq enabling and
remove those redundant parts from the exynos cpuidle drivers

Signed-off-by: Amit Daniel <amit.kachhap@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Robert Lee <rob.lee@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>