PM / Wakeup: Combine atomic counters to avoid reordering issues

The memory barrier in wakeup_source_deactivate() is supposed to
prevent the callers of pm_wakeup_pending() and pm_get_wakeup_count()
from seeing the new value of events_in_progress (0, in particular)
and the old value of event_count at the same time.  However, if
wakeup_source_deactivate() is executed by CPU0 and, for instance,
pm_wakeup_pending() is executed by CPU1, where both processors can
reorder operations, the memory barrier in wakeup_source_deactivate()
doesn't affect CPU1 which can reorder reads.  In that case CPU1 may
very well decide to fetch event_count before it's modified and
events_in_progress after it's been updated, so pm_wakeup_pending()
may fail to detect a wakeup event.  This issue can be addressed by
using a single atomic variable to store both events_in_progress
and event_count, so that they can be updated together in a single
atomic operation.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>