opp: Prepare for ->set_opp() helper to work without regulators

Until now the ->set_opp() helper (i.e. special implementation for
setting the OPPs for platforms) was implemented only to take care of
multiple regulators case, but going forward we would need that for other
use cases as well.

This patch prepares for that by allocating the regulator specific part
from dev_pm_opp_set_regulators() and the opp helper part from
dev_pm_opp_register_set_opp_helper().

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