cpufreq: Manage only online cpus

cpufreq core doesn't manage offline cpus and if driver->init() has
returned
mask including offline cpus, it may result in unwanted behavior by
cpufreq core
or governors.

We need to get only online cpus in this mask. There are two places to
fix this
mask, cpufreq core and cpufreq driver. It makes sense to do this at
common place
and hence is done in core.

Change-Id: If5f0083ab44f808dd0ab23d14a2269ab73119686
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>