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.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>