MAINTAINERS: Add missing git repository and directory for devfreq

This patch adds the missing git repository infomation and directory for
devfreq. When using get_maintainer.pl, the result should include the
correct maintainer information.

Suggested-by: Joe Perches <joe@perches.com>
[cw00.chio: Add missing directory for devfreq]
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>