PM / devfreq: add tracing for scheduling work

This patch add basic tracing of the devfreq workqueue and delayed work.
It aims to capture changes of the polling intervals and device state.

Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>