PM / Domains: Remove reference counting for the generic_pm_domain_data
The reference counting was needed when genpd supported PM domain device
callbacks. Since this option has been removed, let's also remove the
reference counting of the struct generic_pm_domain_data.
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org> Acked-by: Pavel Machek <pavel@ucw.cz> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>