DEVICETREE: Add bindings for the ATH79 DDR controllers

The DDR controller of the ARxxx 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.
Signed-off-by: NAlban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/memory-controllers/ath79-ddr-controller.txt

+Binding for Qualcomm  Atheros AR7xxx/AR9xxx DDR controller
+
+The DDR controller of the ARxxx 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 controllers memory area
+- #qca,ddr-wb-channel-cells: has to be 1, the index of the write buffer
+  channel
+
+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>;
+	};