PM / OPP: Ensure consistent naming of static functions

All exported functions use dev_pm_* prefix and all static functions
are now standardized with _ prefix. This is better than having to deal
with multiple function naming styles within the same file.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>