pm: only define slots if CONFIG_PM is enabled

The pm_device_slots section is only used by subsys/pm/pm.c. This file is
only compiled if CONFIG_PM=y. pm_device_slots is unused otherwise.

Don't populate pm_device_slots when CONFIG_PM=n, CONFIG_PM_DEVICE=y.

Signed-off-by: Marco Widmer <marco.widmer@bytesatwork.ch>