Binding for Qualcomm Atheros AR7xxx/AR9XXX CPU interrupt controller

On most SoC the IRQ controller need to flush the DDR FIFO before running
the interrupt handler of some devices. This is configured using the
qca,ddr-wb-channels and qca,ddr-wb-channel-interrupts properties.

Required Properties:

- compatible: has to be "qca,<soctype>-cpu-intc", "qca,ar7100-cpu-intc"
  as fallback
- interrupt-controller : Identifies the node as an interrupt controller
- #interrupt-cells : Specifies the number of cells needed to encode interrupt
		     source, should be 1 for intc

Please refer to interrupts.txt in this directory for details of the common
Interrupt Controllers bindings used by client devices.

Optional Properties:

- qca,ddr-wb-channel-interrupts: List of the interrupts needing a write
  buffer flush
- qca,ddr-wb-channels: List of phandles to the write buffer channels for
  each interrupt. If qca,ddr-wb-channel-interrupts is not present the interrupt
  default to the entry's index.

Example:

	interrupt-controller {
		compatible = "qca,ar9132-cpu-intc", "qca,ar7100-cpu-intc";

		interrupt-controller;
		#interrupt-cells = <1>;

		qca,ddr-wb-channel-interrupts = <2>, <3>, <4>, <5>;
		qca,ddr-wb-channels = <&ddr_ctrl 3>, <&ddr_ctrl 2>,
					<&ddr_ctrl 0>, <&ddr_ctrl 1>;
	};

	...

	ddr_ctrl: memory-controller@18000000 {
		...
		#qca,ddr-wb-channel-cells = <1>;
	};
