dt-bindings: Add description of Socionext EXIU interrupt controller

Add a description of the External Interrupt Unit (EXIU) interrupt
controller as found on the Socionext SynQuacer SoC.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>

Documentation/devicetree/bindings/interrupt-controller/socionext,synquacer-exiu.txt

+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>;
+	};