cpufreq: export cpufreq_driver_resolve_freq()

Export cpufreq_driver_resolve_freq() since governors may be compiled as
modules.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>