cpufreq: Move to_gov_attr_set() to cpufreq.h

commit ae265086 upstream.

So it can be reused by other codes.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>