genirq: Add wakeup sysfs node to show IRQ wakeup state

Surprisingly there is no simple way to see if the IRQ line in question
is wakeup source or not.

Note that wakeup might be an OOB (out-of-band) source like GPIO line
which makes things slightly more complicated.

Add a sysfs node to cover this case.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Tony Lindgren <tony@atomide.com>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>
Link: https://lkml.kernel.org/r/20180226155043.67937-1-andriy.shevchenko@linux.intel.com

Documentation/ABI/testing/sysfs-kernel-irq

@@ -51,3 +51,10 @@ Date:		September 2016
 KernelVersion:	4.9
 Contact:	Craig Gallek <kraig@google.com>
 Description:	The type of the interrupt.  Either the string 'level' or 'edge'.
+
+What:		/sys/kernel/irq/<irq>/wakeup
+Date:		March 2018
+KernelVersion:	4.17
+Contact:	Andy Shevchenko <andriy.shevchenko@linux.intel.com>
+Description:	The wakeup state of the interrupt. Either the string
+		'enabled' or 'disabled'.