PM / sleep: Report interrupt that caused system wakeup

Add a sysfs attribute, /sys/power/pm_wakeup_irq, reporting the IRQ
number of the first wakeup interrupt (that is, the first interrupt
from an IRQ line armed for system wakeup) seen by the kernel during
the most recent system suspend/resume cycle.

This feature will be useful for system wakeup diagnostics of
spurious wakeup interrupts.

Signed-off-by: Alexandra Yates <alexandra.yates@linux.intel.com>
[ rjw: Fixed up pm_wakeup_irq definition ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/ABI/testing/sysfs-power

@@ -256,3 +256,15 @@ Description:
 		Writing a "1" enables this printing while writing a "0"
 		disables it.  The default value is "0".  Reading from this file
 		will display the current value.
+
+What:		/sys/power/pm_wakeup_irq
+Date:		April 2015
+Contact:	Alexandra Yates <alexandra.yates@linux.intel.org>
+Description:
+		The /sys/power/pm_wakeup_irq file reports to user space the IRQ
+		number of the first wakeup interrupt (that is, the first
+		interrupt from an IRQ line armed for system wakeup) seen by the
+		kernel during the most recent system suspend/resume cycle.
+
+		This output is useful for system wakeup diagnostics of spurious
+		wakeup interrupts.