DT: omap4/5: add binding for the wake-up generator

Add a binding for the OMAP4/5 wake-up generator, which acts as
an interrupt controller feeding into the GIC.

Acked-by: Tony Lindgren <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jason Cooper <[e-mail]>

Documentation/devicetree/bindings/interrupt-controller/ti,omap4-wugen-mpu

···
+TI OMAP4 Wake-up Generator
+
+All TI OMAP4/5 (and their derivatives) an interrupt controller that
+routes interrupts to the GIC, and also serves as a wakeup source. It
+is also referred to as "WUGEN-MPU", hence the name of the binding.
+
+Reguired properties:
+
+- compatible : should contain at least "ti,omap4-wugen-mpu" or
+  "ti,omap5-wugen-mpu"
+- reg : Specifies base physical address and size of the registers.
+- interrupt-controller : Identifies the node as an interrupt controller.
+- #interrupt-cells : Specifies the number of cells needed to encode an
+  interrupt source. The value must be 3.
+- interrupt-parent : a phandle to the GIC these interrupts are routed
+  to.
+
+Notes:
+
+- Because this HW ultimately routes interrupts to the GIC, the
+  interrupt specifier must be that of the GIC.
+- Only SPIs can use the WUGEN as an interrupt parent. SGIs and PPIs
+  are explicitly forbiden.
+
+Example:
+
+       wakeupgen: interrupt-controller@48281000 {
+               compatible = "ti,omap5-wugen-mpu", "ti,omap4-wugen-mpu";
+               interrupt-controller;
+               #interrupt-cells = <3>;
+               reg = <0x48281000 0x1000>;
+               interrupt-parent = <&gic>;
+       };