Binding for Qualcomm  Atheros AR7xxx/AR9xxx DDR controller

The DDR controller of the AR7xxx and AR9xxx families provides an interface
to flush the FIFO between various devices and the DDR. This is mainly used
by the IRQ controller to flush the FIFO before running the interrupt handler
of such devices.

Required properties:

- compatible: has to be "qca,<soc-type>-ddr-controller",
  "qca,[ar7100|ar7240]-ddr-controller" as fallback.
  On SoC with PCI support "qca,ar7100-ddr-controller" should be used as
  fallback, otherwise "qca,ar7240-ddr-controller" should be used.
- reg: Base address and size of the controller's memory area
- #qca,ddr-wb-channel-cells: Specifies the number of cells needed to encode
			     the write buffer channel index, should be 1.

Example:

	ddr_ctrl: memory-controller@18000000 {
		compatible = "qca,ar9132-ddr-controller",
				"qca,ar7240-ddr-controller";
		reg = <0x18000000 0x100>;

		#qca,ddr-wb-channel-cells = <1>;
	};

	...

	interrupt-controller {
		...
		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>;
	};
