irqchip: xtensa-pic: xtensa-mx: document DT bindings

* Xtensa Interrupt Distributor and Programmable Interrupt Controller (MX)

Required properties:

- compatible: Should be "cdns,xtensa-mx".

Remaining properties have exact same meaning as in Xtensa PIC

(see cdns,xtensa-pic.txt).

Examples:

	pic: pic {

		compatible = "cdns,xtensa-mx";

		/* one cell: internal irq number,

		 * two cells: second cell == 0: internal irq number

		 *            second cell == 1: external irq number

		 */

		#interrupt-cells = <2>;

		interrupt-controller;

	};

* Xtensa built-in Programmable Interrupt Controller (PIC)

Required properties:

- compatible: Should be "cdns,xtensa-pic".

- interrupt-controller: Identifies the node as an interrupt controller.

- #interrupt-cells: The number of cells to define the interrupts.

  It may be either 1 or 2.

  When it's 1, the first cell is the internal IRQ number.

  When it's 2, the first cell is the IRQ number, and the second cell

  specifies whether it's internal (0) or external (1).

  Periferals are usually connected to a fixed external IRQ, but for different

  core variants it may be mapped to different internal IRQ.

  IRQ sensitivity and priority are fixed for each core variant and may not be

  changed at runtime.

Examples:

	pic: pic {

		compatible = "cdns,xtensa-pic";

		/* one cell: internal irq number,

		 * two cells: second cell == 0: internal irq number

		 *            second cell == 1: external irq number

		 */

		#interrupt-cells = <2>;

		interrupt-controller;

	};