PM: domains: Implement the ->set_performance_state() callback for genpd

To enable generic support for performance scaling for PM domains, let's
implement the ->set_performance_state() callback for genpd.

Beyond this change, users of the corresponding genpd specific API,
dev_pm_genpd_set_performance_state() are encouraged to switch to the common
dev_pm_domain_set_performance_state() API.

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