cpufreq: mvebu: Move cpufreq code into drivers/cpufreq/

Move cpufreq bits for mvebu into drivers/cpufreq/ directory, that's
where they really belong to.

Compiled tested only.

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