cpufreq: remove at32ap-cpufreq

Since AVR32 arch was removed, at32ap-cpufreq is useless.
Remove this driver.

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