dt-bindings: interrupt-controller: Add DT binding for meson GPIO interrupt controller

This commit adds the device tree bindings description for Amlogic's GPIO
interrupt controller available on the meson8b, gxbb and gxl SoC families

Cc: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

Documentation/devicetree/bindings/interrupt-controller/amlogic,meson-gpio-intc.txt

+Amlogic meson GPIO interrupt controller
+
+Meson SoCs contains an interrupt controller which is able to watch the SoC
+pads and generate an interrupt on edge or level. The controller is essentially
+a 256 pads to 8 GIC interrupt multiplexer, with a filter block to select edge
+or level and polarity. It does not expose all 256 mux inputs because the
+documentation shows that the upper part is not mapped to any pad. The actual
+number of interrupt exposed depends on the SoC.
+
+Required properties:
+
+- compatible : must have "amlogic,meson8-gpio-intc” and either
+   “amlogic,meson8b-gpio-intc” for meson8b SoCs (S805) or
+   “amlogic,meson-gxbb-gpio-intc” for GXBB SoCs (S905) or
+   “amlogic,meson-gxl-gpio-intc” for GXL SoCs (S905X, S912)
+- interrupt-parent : a phandle to the GIC the interrupts are routed to.
+   Usually this is provided at the root level of the device tree as it is
+   common to most of the SoC.
+- reg : Specifies base physical address and size of the 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 2.
+- meson,channel-interrupts: Array with the 8 upstream hwirq numbers. These
+   are the hwirqs used on the parent interrupt controller.
+
+Example:
+
+gpio_interrupt: interrupt-controller@9880 {
+	compatible = "amlogic,meson-gxbb-gpio-intc",
+		     "amlogic,meson-gpio-intc";
+	reg = <0x0 0x9880 0x0 0x10>;
+	interrupt-controller;
+	#interrupt-cells = <2>;
+	meson,channel-interrupts = <64 65 66 67 68 69 70 71>;
+};