PM / OPP: Implement dev_pm_opp_get_of_node()

This adds a new helper to let the power domain drivers to access
opp->np, so that they can read platform specific properties from the
node.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>