dt-bindings: dmaengine: Add MediaTek High-Speed DMA controller bindings

Document the devicetree bindings for MediaTek High-Speed DMA controller
which could be found on MT7623 SoC or other similar Mediatek SoCs.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>