cpufreq: exynos: simplify .init() for setting policy->cpus

With the recent changes in cpufreq core, we just need to set mask of all
possible cpus into policy->cpus. Rest would be done by core.

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