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>

drivers/devfreq/Kconfig

 menuconfig PM_DEVFREQ
 	bool "Generic Dynamic Voltage and Frequency Scaling (DVFS) support"
 	select SRCU
+	select PM_OPP
 	help
 	  A device may have a list of frequencies and voltages available.
 	  devfreq, a generic DVFS framework can be registered for a device