STM32 External Interrupt Controller

Required properties:

- compatible: Should be "st,stm32-exti"
- reg: Specifies base physical address and size of the registers
- interrupt-controller: Indentifies the node as an interrupt controller
- #interrupt-cells: Specifies the number of cells to encode an interrupt
  specifier, shall be 2
- interrupts: interrupts references to primary interrupt controller

Example:

exti: interrupt-controller@40013c00 {
	compatible = "st,stm32-exti";
	interrupt-controller;
	#interrupt-cells = <2>;
	reg = <0x40013C00 0x400>;
	interrupts = <1>, <2>, <3>, <6>, <7>, <8>, <9>, <10>, <23>, <40>, <41>, <42>, <62>, <76>;
};
