dt-bindings: dmaengine: add DT binding for UniPhier MIO DMAC

The MIO DMAC (Media IO DMA Controller) is used in UniPhier LD4,
Pro4, and sLD8 SoCs.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>