Synopsys DesignWare APB interrupt controller (dw_apb_ictl)

Synopsys DesignWare provides interrupt controller IP for APB known as
dw_apb_ictl. The IP is used as secondary interrupt controller in some SoCs with
APB bus, e.g. Marvell Armada 1500.

Required properties:
- compatible: shall be "snps,dw-apb-ictl"
- reg: physical base address of the controller and length of memory mapped
  region starting with ENABLE_LOW register
- interrupt-controller: identifies the node as an interrupt controller
- #interrupt-cells: number of cells to encode an interrupt-specifier, shall be 1
- interrupts: interrupt reference to primary interrupt controller
- interrupt-parent: (optional) reference specific primary interrupt controller

The interrupt sources map to the corresponding bits in the interrupt
registers, i.e.
- 0 maps to bit 0 of low interrupts,
- 1 maps to bit 1 of low interrupts,
- 32 maps to bit 0 of high interrupts,
- 33 maps to bit 1 of high interrupts,
- (optional) fast interrupts start at 64.

Example:
	aic: interrupt-controller@3000 {
		compatible = "snps,dw-apb-ictl";
		reg = <0x3000 0xc00>;
		interrupt-controller;
		#interrupt-cells = <1>;
		interrupt-parent = <&gic>;
		interrupts = <GIC_SPI 3 IRQ_TYPE_LEVEL_HIGH>;
	};
