PM / devfreq: Move statistics to separate struct devfreq_stats

Count time and transitions between devfreq frequencies in separate
struct devfreq_stats for improved code readability and maintenance.

Signed-off-by: Kamil Konieczny <k.konieczny@samsung.com>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
[cw00.choi: Fix the merge conflict in trasn_stat_store
 and use 'devfreq->stats.*' style for consistent coding style
 and restore the clean-up code of 'devfreq->profile->*']
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>