dt-bindings: dmaengine: Add Spreadtrum SC9860 DMA controller

This patch adds the binding documentation for Spreadtrum SC9860 DMA
controller device.

Signed-off-by: Baolin Wang <baolin.wang@spreadtrum.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>