OPP: Introduce an OF helper function to inform if required-opps is used

As being shown from a subsequent change to genpd, it's useful to understand
if a device's OF node has an OPP-table described and whether it contains
OPP nodes that makes use of the required-opps DT property.

For this reason, let's introduce an OPP OF helper function called
dev_pm_opp_of_has_required_opp().

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>