PM / domains: Add perf_state attribute to genpd debugfs

Now that genpd supports performance states, add this additional
attribute as part of the power domains debugfs entry, to display
the current performance state for the Power domain.

Suggested-by: David Collins <collinsd@codeaurora.org>
Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>