cpufreq: Move governor attribute set headers to cpufreq.h

Move definitions and function headers related to struct gov_attr_set
to include/linux/cpufreq.h so they can be used by (future) goverernors
located outside of drivers/cpufreq/.

No functional changes.

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