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: NDaniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: NViresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: NBartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: NTomasz Figa <t.figa@samsung.com>
Signed-off-by: NKukjin Kim <kgene.kim@samsung.com>