cpufreq: AVR32: move cpufreq driver to drivers/cpufreq

This patch moves cpufreq driver of AVR32 based at32ap platform to
drivers/cpufreq.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>