dt-bindings/irq: Add binding for Freescale IRQSTEER multiplexer

Freescale IRQSTEER Interrupt multiplexer

Required properties:

- compatible: should be:

	- "fsl,imx8m-irqsteer"

	- "fsl,imx-irqsteer"

- reg: Physical base address and size of registers.

- interrupts: Should contain the parent interrupt line used to multiplex the

  input interrupts.

- clocks: Should contain one clock for entry in clock-names

  see Documentation/devicetree/bindings/clock/clock-bindings.txt

- clock-names:

   - "ipg": main logic clock

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

- #interrupt-cells: Specifies the number of cells needed to encode an

  interrupt source. The value must be 1.

- fsl,channel: The output channel that all input IRQs should be steered into.

- fsl,irq-groups: Number of IRQ groups managed by this controller instance.

  Each group manages 64 input interrupts.

Example:

	interrupt-controller@32e2d000 {

		compatible = "fsl,imx8m-irqsteer", "fsl,imx-irqsteer";

		reg = <0x32e2d000 0x1000>;

		interrupts = <GIC_SPI 18 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&clk IMX8MQ_CLK_DISP_APB_ROOT>;

		clock-names = "ipg";

		fsl,channel = <0>;

		fsl,irq-groups = <1>;

		interrupt-controller;

		#interrupt-cells = <1>;

	};