cpufreq: ARM big LITTLE: Move cpu_to_cluster() to arm_big_little.h

The cpu_to_cluster() function may be used by glue drivers, so it's
better to keep it in arm_big_little.h.

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