Xilinx AXI VDMA engine, it does transfers between memory and video devices.
It can be configured to have one channel or two channels. If configured
as two channels, one is to transmit to the video device and another is
to receive from the video device.

Required properties:
- compatible: Should be "xlnx,axi-vdma"
- reg: Should contain VDMA registers location and length.
- interrupts: Should contain per channel VDMA interrupts.
- compatible (child node): It should be either "xlnx,axi-vdma-mm2s-channel" or
	"xlnx,axi-vdma-s2mm-channel". It depends on the hardware design and it
	can also have both channels.

Example:
++++++++

axi_vdma_0: axivdma@40030000 {
	compatible = "xlnx,axi-vdma";
	reg = < 0x40030000 0x10000 >;
	dma-channel@40030000 {
		compatible = "xlnx,axi-vdma-mm2s-channel";
		interrupts = < 0 54 4 >;
	} ;
	dma-channel@40030030 {
		compatible = "xlnx,axi-vdma-s2mm-channel";
		interrupts = < 0 53 4 >;
	} ;
} ;
