* Advanced Interrupt Controller (AIC)

Required properties:
- compatible: Should be "atmel,<chip>-aic"
- interrupt-controller: Identifies the node as an interrupt controller.
- interrupt-parent: For single AIC system, it is an empty property.
- #interrupt-cells: The number of cells to define the interrupts. It sould be 3.
  The first cell is the IRQ number (aka "Peripheral IDentifier" on datasheet).
  The second cell is used to specify flags:
    bits[3:0] trigger type and level flags:
      1 = low-to-high edge triggered.
      2 = high-to-low edge triggered.
      4 = active high level-sensitive.
      8 = active low level-sensitive.
      Valid combinations are 1, 2, 3, 4, 8.
      Default flag for internal sources should be set to 4 (active high).
  The third cell is used to specify the irq priority from 0 (lowest) to 7
  (highest).
- reg: Should contain AIC registers location and length
- atmel,external-irqs: u32 array of external irqs.

Examples:
	/*
	 * AIC
	 */
	aic: interrupt-controller@fffff000 {
		compatible = "atmel,at91rm9200-aic";
		interrupt-controller;
		interrupt-parent;
		#interrupt-cells = <3>;
		reg = <0xfffff000 0x200>;
	};

	/*
	 * An interrupt generating device that is wired to an AIC.
	 */
	dma: dma-controller@ffffec00 {
		compatible = "atmel,at91sam9g45-dma";
		reg = <0xffffec00 0x200>;
		interrupts = <21 4 5>;
	};