MAINTAINERS: Add devfreq-event entry

This patch adds the devfreq-event[1] entry to review the patches
as just supporter. Patches will be picked up by Myungjoo Ham on
devfreq git repository.
[1] https://lkml.org/lkml/2015/1/25/573

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>