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

* 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>;

	};