cpufreq: Implement cpufreq_generic_suspend()

Multiple platforms need to set CPUs to a particular frequency before
suspending the system, so provide a common infrastructure for them.

Those platforms only need to point their ->suspend callback pointers
to the generic routine.

Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
[rjw: Changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>