MAINTAINERS: Update myself as maintainer for DEVFREQ subsystem support

Update myself to the DEVFREQ entry as maintainer from reviewer and
the git repository information to manage the devfreq patches. I've been
reviewing and tesing the devfreq support for the couple of years as reviewer.
>From now, I'll help and reiview the devfreq as maintainer.

Suggested-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>