MIPS CPU interrupt controller

On MIPS the mips_cpu_irq_of_init() helper can be used to initialize the 8 CPU
IRQs from a devicetree file and create a irq_domain for IRQ controller.

With the irq_domain in place we can describe how the 8 IRQs are wired to the
platforms internal interrupt controller cascade.

Below is an example of a platform describing the cascade inside the devicetree
and the code used to load it inside arch_init_irq().

Required properties:
- compatible : Should be "mti,cpu-interrupt-controller"

Example devicetree:
	cpu-irq: cpu-irq@0 {
		#address-cells = <0>;

		interrupt-controller;
		#interrupt-cells = <1>;

		compatible = "mti,cpu-interrupt-controller";
	};

	intc: intc@200 {
		compatible = "ralink,rt2880-intc";
		reg = <0x200 0x100>;

		interrupt-controller;
		#interrupt-cells = <1>;

		interrupt-parent = <&cpu-irq>;
		interrupts = <2>;
	};


Example platform irq.c:
static struct of_device_id __initdata of_irq_ids[] = {
	{ .compatible = "mti,cpu-interrupt-controller", .data = mips_cpu_irq_of_init },
	{ .compatible = "ralink,rt2880-intc", .data = intc_of_init },
	{},
};

void __init arch_init_irq(void)
{
	of_irq_init(of_irq_ids);
}