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;
        };
    };
