dt-bindings: milbeaut-m10v-hdmac: Add Socionext Milbeaut HDMAC bindings

Document the devicetree bindings for Socionext Milbeaut HDMAC
controller. Controller has upto 8 floating channels, that need
a predefined slave-id to work from a set of slaves.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Link: https://lore.kernel.org/r/20191015033350.14866-1-jassisinghbrar@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>