cpufreq: blackfin: move cpufreq driver to drivers/cpufreq

This patch moves cpufreq driver of BLACKFIN architecture to drivers/cpufreq.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Steven Miao <realmz6@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>