PM / devfreq: Move struct devfreq_governor to devfreq directory

This patch moves the struct devfreq_governor from header file
to the devfreq directory because this structure is private data
and it have to be only accessed by the devfreq core.

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