dt-bindings: interrupt-controller: Add Aspeed SCU interrupt controller

Document the Aspeed SCU interrupt controller and add an include file
for the interrupts it provides.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/1579123790-6894-2-git-send-email-eajames@linux.ibm.com

Documentation/devicetree/bindings/interrupt-controller/aspeed,ast2xxx-scu-ic.txt

@@ -0,0 +1,23 @@
+Aspeed AST25XX and AST26XX SCU Interrupt Controller
+
+Required Properties:
+ - #interrupt-cells		: must be 1
+ - compatible			: must be "aspeed,ast2500-scu-ic",
+				  "aspeed,ast2600-scu-ic0" or
+				  "aspeed,ast2600-scu-ic1"
+ - interrupts			: interrupt from the parent controller
+ - interrupt-controller		: indicates that the controller receives and
+				  fires new interrupts for child busses
+
+Example:
+
+    syscon@1e6e2000 {
+        ranges = <0 0x1e6e2000 0x1a8>;
+
+        scu_ic: interrupt-controller@18 {
+            #interrupt-cells = <1>;
+            compatible = "aspeed,ast2500-scu-ic";
+            interrupts = <21>;
+            interrupt-controller;
+        };
+    };