of/irq: Document properties for wakeup interrupt parent

Some interrupt controllers in a SoC, are always powered on and have a
select interrupts routed to them, so that they can wakeup the SoC from
suspend. Add wakeup-parent DT property to refer to these interrupt
controllers.

Signed-off-by: Lina Iyer <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Reviewed-by: Stephen Boyd <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/interrupt-controller/interrupts.txt

···
 			sensitivity = <7>;
 		};
 	};
+
+3) Interrupt wakeup parent
+--------------------------
+
+Some interrupt controllers in a SoC, are always powered on and have a select
+interrupts routed to them, so that they can wakeup the SoC from suspend. These
+interrupt controllers do not fall into the category of a parent interrupt
+controller and can be specified by the "wakeup-parent" property and contain a
+single phandle referring to the wakeup capable interrupt controller.
+
+   Example:
+	wakeup-parent = <&pdc_intc>;