* ARM Nested Vector Interrupt Controller (NVIC)

The NVIC provides an interrupt controller that is tightly coupled to
Cortex-M based processor cores.  The NVIC implemented on different SoCs
vary in the number of interrupts and priority bits per interrupt.

Main node required properties:

- compatible : should be one of:
	"arm,v6m-nvic"
	"arm,v7m-nvic"
	"arm,v8m-nvic"
- interrupt-controller : Identifies the node as an interrupt controller
- #interrupt-cells : Specifies the number of cells needed to encode an
  interrupt source.  The type shall be a <u32> and the value shall be 2.

  The 1st cell contains the interrupt number for the interrupt type.

  The 2nd cell is the priority of the interrupt.

- reg : Specifies base physical address(s) and size of the NVIC registers.
  This is at a fixed address (0xe000e100) and size (0xc00).

- arm,num-irq-priority-bits: The number of priority bits implemented by the
  given SoC

Example:

	intc: interrupt-controller@e000e100 {
		compatible = "arm,v7m-nvic";
		#interrupt-cells = <2>;
		#address-cells = <1>;
		interrupt-controller;
		reg = <0xe000e100 0xc00>;
		arm,num-irq-priority-bits = <4>;
	};