PM / wakeup: Export wakeup_count instead of event_count via sysfs

Currently we export event_count instead of wakeup_count via the
per-device wakeup_count sysfs attribute. Change it to wakeup_count
to make it more meaningful.

wakeup_count increments only when events_check_enabled is set,
that is whenever writes the current wakeup count to
/sys/power/wakeup_count.  Also events_check_enabled is cleared on
every resume. User space is expected to write to this just before
suspend.  This way pm_wakeup_event(), when called from IRQs handles,
will increment wakeup_count only if we are in system-wide
suspend-resume cycle and should give a fair approximation of how many
times a device may have triggered a wakeup from system suspend.

event_count on the other hand will increment every time
pm_wakeup_event() is called irrespective of whether we are in a
suspend-resume cycle and some drivers call it on every interrupt
which makes it less useful for system wakeup tracking.

Signed-off-by: Ravi Chandra Sadineni <ravisadineni@chromium.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>