dt-bindings: Add description of Socionext EXIU interrupt controller

Socionext SynQuacer External Interrupt Unit (EXIU)

The Socionext Synquacer SoC has an external interrupt unit (EXIU)

that forwards a block of 32 configurable input lines to 32 adjacent

level-high type GICv3 SPIs.

Required properties:

- compatible           : Should be "socionext,synquacer-exiu".

- reg                  : Specifies base physical address and size of the

                         control registers.

- 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 3.

- interrupt-parent     : phandle of the GIC these interrupts are routed to.

- socionext,spi-base   : The SPI number of the first SPI of the 32 adjacent

                         ones the EXIU forwards its interrups to.

Notes:

- Only SPIs can use the EXIU as an interrupt parent.

Example:

	exiu: interrupt-controller@510c0000 {

		compatible = "socionext,synquacer-exiu";

		reg = <0x0 0x510c0000 0x0 0x20>;

		interrupt-controller;

		interrupt-parent = <&gic>;

		#interrupt-cells = <3>;

		socionext,spi-base = <112>;

	};