MIPS Global Interrupt Controller (GIC)

The MIPS GIC routes external interrupts to individual VPEs and IRQ pins.
It also supports local (per-processor) interrupts and software-generated
interrupts which can be used as IPIs.  The GIC also includes a free-running
global timer, per-CPU count/compare timers, and a watchdog.

Required properties:
- compatible : Should be "mti,gic".
- interrupt-controller : Identifies the node as an interrupt controller
- #interrupt-cells : Specifies the number of cells needed to encode an
  interrupt specifier.  Should be 3.
  - The first cell is the type of interrupt, local or shared.
    See <include/dt-bindings/interrupt-controller/mips-gic.h>.
  - The second cell is the GIC interrupt number.
  - The third cell encodes the interrupt flags.
    See <include/dt-bindings/interrupt-controller/irq.h> for a list of valid
    flags.

Optional properties:
- reg : Base address and length of the GIC registers.  If not present,
  the base address reported by the hardware GCR_GIC_BASE will be used.
- mti,reserved-cpu-vectors : Specifies the list of CPU interrupt vectors
  to which the GIC may not route interrupts.  Valid values are 2 - 7.
  This property is ignored if the CPU is started in EIC mode.

Required properties for timer sub-node:
- compatible : Should be "mti,gic-timer".
- interrupts : Interrupt for the GIC local timer.

Optional properties for timer sub-node:
- clocks : GIC timer operating clock.
- clock-frequency : Clock frequency at which the GIC timers operate.

Note that one of clocks or clock-frequency must be specified.

Example:

	gic: interrupt-controller@1bdc0000 {
		compatible = "mti,gic";
		reg = <0x1bdc0000 0x20000>;

		interrupt-controller;
		#interrupt-cells = <3>;

		mti,reserved-cpu-vectors = <7>;

		timer {
			compatible = "mti,gic-timer";
			interrupts = <GIC_LOCAL 1 IRQ_TYPE_NONE>;
			clock-frequency = <50000000>;
		};
	};

	uart@18101400 {
		...
		interrupt-parent = <&gic>;
		interrupts = <GIC_SHARED 24 IRQ_TYPE_LEVEL_HIGH>;
		...
	};
