ARM: S3C64XX: use timekeeping wrapper on cpuidle

The timekeeping is computed from the cpuidle core if we set
the .en_core_tk_irqen flag. Let's use it and remove the duplicated
code.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>