PM / devfreq: Add dependency on PM_OPP

The devfreq ues the OPP library to handle the voltage and frequency
for the device basically. This patch adds the dependency on CONFIG_PM_OPP
in order to prevent either the build break or the unknow behavior.

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