Sigma Designs SMP86xx/SMP87xx secondary interrupt controller

Required properties:
- compatible: should be "sigma,smp8642-intc"
- reg: physical address of MMIO region
- ranges: address space mapping of child nodes
- interrupt-controller: boolean
- #address-cells: should be <1>
- #size-cells: should be <1>

One child node per control block with properties:
- reg: address of registers for this control block
- interrupt-controller: boolean
- #interrupt-cells: should be <2>, interrupt index and flags per interrupts.txt
- interrupts: interrupt spec of primary interrupt controller

Example:

interrupt-controller@6e000 {
	compatible = "sigma,smp8642-intc";
	reg = <0x6e000 0x400>;
	ranges = <0x0 0x6e000 0x400>;
	interrupt-parent = <&gic>;
	interrupt-controller;
	#address-cells = <1>;
	#size-cells = <1>;

	irq0: interrupt-controller@0 {
		reg = <0x000 0x100>;
		interrupt-controller;
		#interrupt-cells = <2>;
		interrupts = <GIC_SPI 2 IRQ_TYPE_LEVEL_HIGH>;
	};

	irq1: interrupt-controller@100 {
		reg = <0x100 0x100>;
		interrupt-controller;
		#interrupt-cells = <2>;
		interrupts = <GIC_SPI 3 IRQ_TYPE_LEVEL_HIGH>;
	};

	irq2: interrupt-controller@300 {
		reg = <0x300 0x100>;
		interrupt-controller;
		#interrupt-cells = <2>;
		interrupts = <GIC_SPI 4 IRQ_TYPE_LEVEL_HIGH>;
	};
};