opp: Implement dev_pm_opp_set_opp()

The new helper dev_pm_opp_set_opp() can be used for configuring the
devices for a particular OPP and can be used by different type of
devices, even the ones which don't change frequency (like power
domains).

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Dmitry Osipenko <digetx@gmail.com>