PM / devfreq: Cache OPP table reference in devfreq

The OPP table can be used often in devfreq. Trying to get it each time can
be expensive, so cache it in the devfreq struct.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
[ Viresh: Added a blank line ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>