PM: <linux/device.h>: fix @em_pd kernel-doc warning

Fix kernel-doc warning in <linux/device.h>:

../include/linux/device.h:613: warning: Function parameter or member 'em_pd' not described in 'device'

Fixes: 1bc138c6 ("PM / EM: add support for other devices than CPUs in Energy Model")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/linux/device.h

@@ -454,6 +454,7 @@ struct dev_links_info {
  * @pm_domain:	Provide callbacks that are executed during system suspend,
  * 		hibernation, system resume and during runtime PM transitions
  * 		along with subsystem-level and driver-level callbacks.
+ * @em_pd:	device's energy model performance domain
  * @pins:	For device pin management.
  *		See Documentation/driver-api/pinctl.rst for details.
  * @msi_list:	Hosts MSI descriptors