MAINTAINERS: Add myself as reviewer for DEVFREQ subsystem support

Add myself to the DEVFREQ entry as reviewer, I've been helping reviewing
and tesing the devfreq support patches for the couple of years. Also,
I'm going to contribute the improvement for devfreq subsystem. It would
be easier for me for review if I'm cc'ed for patches.

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>