irqchip: DT bindings for Cortina Gemini irqchip

This adds device tree bindings for the Cortina Gemini interrupt
controller. They are pretty standard.

Cc: Janos Laube <janos.dev@gmail.com>
Cc: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

Documentation/devicetree/bindings/interrupt-controller/cortina,gemini-interrupt-controller.txt

+* Cortina Systems Gemini interrupt controller
+
+This interrupt controller is found on the Gemini SoCs.
+
+Required properties:
+- compatible: must be "cortina,gemini-interrupt-controller"
+- reg: The register bank for the interrupt controller.
+- interrupt-controller: Identifies the node as an interrupt controller
+- #interrupt-cells: The number of cells to define the interrupts.
+  Must be 2 as the controller can specify level or rising edge
+  IRQs. The bindings follows the standard binding for controllers
+  with two cells specified in
+  interrupt-controller/interrupts.txt
+
+Example:
+
+interrupt-controller@48000000 {
+	compatible = "cortina,gemini-interrupt-controller";
+	reg = <0x48000000 0x1000>;
+	interrupt-controller;
+	#interrupt-cells = <2>;
+};