cpufreq: vexpress-spc: Switch cpumask from topology core to OPP sharing

Since commit ca74b316 ("arm: Use common cpu_topology structure and
functions.") the core cpumask has to be modified during cpu hotplug
operations. So using them to set up cpufreq policy cpumask may be
incorrect as it may contain only cpus that are online at that instance.

Instead, we can use the cpumask setup by OPP library that contains all
the cpus sharing OPP table using dev_pm_opp_get_sharing_cpus.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>