PM / OPP: Export more symbols for module usage

Export cpufreq helpers in OPP to make the cpufreq-core0 and highbank-cpufreq
drivers loadable as modules.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
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>