dt-bindings: dmaengine: sf-pdma: add bindins for SiFive PDMA

Add DT bindings document for Platform DMA(PDMA) driver of board,
HiFive Unleashed Rev A00.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Signed-off-by: Green Wan <green.wan@sifive.com>
Link: https://lore.kernel.org/r/20191107084955.7580-2-green.wan@sifive.com


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