dt-bindings: milbeaut-m10v-xdmac: Add Socionext Milbeaut XDMAC bindings

Document the devicetree bindings for Socionext Milbeaut XDMAC
controller. Controller only supports Mem->Mem transfers. Number
of physical channels are determined by the number of irqs registered.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Link: https://lore.kernel.org/r/20191015033157.14656-1-jassisinghbrar@gmail.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/milbeaut-m10v-xdmac.txt

+* Milbeaut AXI DMA Controller
+
+Milbeaut AXI DMA controller has only memory to memory transfer capability.
+
+* DMA controller
+
+Required property:
+- compatible: 	Should be  "socionext,milbeaut-m10v-xdmac"
+- reg:		Should contain DMA registers location and length.
+- interrupts: 	Should contain all of the per-channel DMA interrupts.
+                Number of channels is configurable - 2, 4 or 8, so
+                the number of interrupts specified should be {2,4,8}.
+- #dma-cells: 	Should be 1.
+
+Example:
+	xdmac0: dma-controller@1c250000 {
+		compatible = "socionext,milbeaut-m10v-xdmac";
+		reg = <0x1c250000 0x1000>;
+		interrupts = <0 17 0x4>,
+			     <0 18 0x4>,
+			     <0 19 0x4>,
+			     <0 20 0x4>;
+		#dma-cells = <1>;
+	};