dt-bindings: interrupt-controller: Add Amazon's Annapurna Labs FIC

Document Amazon's Annapurna Labs Fabric Interrupt Controller SoC binding.
Signed-off-by: Talel Shenhar <talel@amazon.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

Documentation/devicetree/bindings/interrupt-controller/amazon,al-fic.txt

+Amazon's Annapurna Labs Fabric Interrupt Controller
+
+Required properties:
+
+- compatible: should be "amazon,al-fic"
+- reg: physical base address and size of the registers
+- interrupt-controller: identifies the node as an interrupt controller
+- #interrupt-cells: must be 2.
+  First cell defines the index of the interrupt within the controller.
+  Second cell is used to specify the trigger type and must be one of the
+  following:
+    - bits[3:0] trigger type and level flags
+	1 = low-to-high edge triggered
+	4 = active high level-sensitive
+- interrupt-parent: specifies the parent interrupt controller.
+- interrupts: describes which input line in the interrupt parent, this
+  fic's output is connected to. This field property depends on the parent's
+  binding
+
+Example:
+
+amazon_fic: interrupt-controller@0xfd8a8500 {
+	compatible = "amazon,al-fic";
+	interrupt-controller;
+	#interrupt-cells = <2>;
+	reg = <0x0 0xfd8a8500 0x0 0x1000>;
+	interrupt-parent = <&gic>;
+	interrupts = <GIC_SPI 0x0 IRQ_TYPE_LEVEL_HIGH>;
+};