cpufreq: Create for_each_governor()

To make code more readable and less error prone, lets create a helper macro for
iterating over all available governors.

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