opp: Allow _generic_set_opp_regulator() to work for non-freq devices

The _generic_set_opp_regulator() helper will be used for devices which
don't change frequency (like power domains, etc.) later on, prepare for
that by not relying on frequency for making decisions here.

While at it, update its parameters to pass only what is necessary.

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