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>