ARM: EXYNOS: Use the cpu_pm notifier for pm

Use the cpu_pm_enter/exit notifier to group some pm code inside the
pm file.

The save and restore code is duplicated across pm.c and cpuidle.c. By
using the cpu_pm notifier, we can factor out the routine.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>