OPP: Add dev_pm_opp_get_supplies()

We already have an API for getting voltage information for a single
regulator, dev_pm_opp_get_voltage(), but there is nothing available for
multiple regulator case.

This patch adds a new API, dev_pm_opp_get_supplies(), to get all
information related to the supplies for an OPP.

Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>