PM / Domains: export symbols to add/remove devices from genpd

Drivers which can be built as loadable module needs symbols
- pm_genpd_add_device/pm_genpd_remove_device to add/remove devices
to/from genpd. Those drivers create platform devices, which comes
under a powerdomain.

Signed-off-by: Maruthi Bayyavarapu <maruthi.bayyavarapu@amd.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>