dmaengine: plx-dma: Implement descriptor submission

dmaengine: plx-dma: Implement descriptor submission

On prep, a spin lock is taken and the next entry in the circular buffer
is filled. On submit, the valid bit is set in the hardware descriptor
and the lock is released.

The DMA engine is started (if it's not already running) when the client
calls dma_async_issue_pending().

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Link: https://lore.kernel.org/r/20200103212021.2881-4-logang@deltatee.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>