PM / devfreq: Set min/max_freq when adding the devfreq device

Prior to that, the min/max_freq of the devfreq device are always zero
before the user changes the min/max_freq through sysfs entries.
It might make the confusion for the min/max_freq.

This patch initializes the available min/max_freq by using the OPP
during adding the devfreq device.

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