cpufreq: Add cpufreq_cpu_acquire() and cpufreq_cpu_release()

It sometimes is necessary to find a cpufreq policy for a given CPU
and acquire its rwsem (for writing) immediately after that, so
introduce cpufreq_cpu_acquire() as a helper for that and the
complementary cpufreq_cpu_release().

Make cpufreq_update_policy() use the new functions.

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