PM / OPP: Expose _of_get_opp_desc_node as dev_pm_opp API

Rename _of_get_opp_desc_node to dev_pm_opp_of_get_opp_desc_node and add it
to include/linux/pm_opp.h to allow other drivers, such as platform OPP
and cpufreq drivers, to make use of it.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>