PM / devfreq: Move private devfreq_update_stats() into devfreq

THe devfreq_update_stats() updates the 'struct devfreq_dev_status'
in order to get current status of devfreq device. It is only used
for the governors.

This patch moves the devfreq_update_stats() into devfreq directory.

Signed-off-by: Chanwoo Choi <cwchoi00@gmail.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>