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>;
       };