trace: events: add devfreq trace event file

The patch adds a new file for with trace events for devfreq
framework. They are used for performance analysis of the framework.
It also contains updates in MAINTAINERS file adding new entry for
devfreq maintainers.

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>