* ARM Versatile FPGA interrupt controller

One or more FPGA IRQ controllers can be synthesized in an ARM reference board
such as the Integrator or Versatile family. The output of these different
controllers are OR:ed together and fed to the CPU tile's IRQ input. Each
instance can handle up to 32 interrupts.

Required properties:
- compatible: "arm,versatile-fpga-irq"
- interrupt-controller: Identifies the node as an interrupt controller
- #interrupt-cells: The number of cells to define the interrupts.  Must be 1
  as the FPGA IRQ controller has no configuration options for interrupt
  sources.  The cell is a u32 and defines the interrupt number.
- reg: The register bank for the FPGA interrupt controller.
- clear-mask: a u32 number representing the mask written to clear all IRQs
  on the controller at boot for example.
- valid-mask: a u32 number representing a bit mask determining which of
  the interrupts are valid. Unconnected/unused lines are set to 0, and
  the system till not make it possible for devices to request these
  interrupts.

Example:

pic: pic@14000000 {
        compatible = "arm,versatile-fpga-irq";
        #interrupt-cells = <1>;
        interrupt-controller;
        reg = <0x14000000 0x100>;
        clear-mask = <0xffffffff>;
        valid-mask = <0x003fffff>;
};

Optional properties:
- interrupts: if the FPGA IRQ controller is cascaded, i.e. if its IRQ
  output is simply connected to the input of another IRQ controller,
  then the parent IRQ shall be specified in this property.
