PM / OPP: switch exported symbols to GPL variant

We are GPLV2 library, so be clear in the symbols exported as well.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>