cpufreq: Add of_perf_domain_get_sharing_cpumask

Add of_perf_domain_get_sharing_cpumask function to group cpu
to specific performance domain.

Signed-off-by: Hector.Yuan <hector.yuan@mediatek.com>
[ Viresh: create separate routine parse_perf_domain() and always set the
	  cpumask. ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>