dmaengine: plx-dma: Introduce PLX DMA engine PCI driver skeleton

Some PLX Switches can expose DMA engines via extra PCI functions
on the upstream port. Each function will have one DMA channel.

This patch is just the core PCI driver skeleton and dma
engine registration.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Link: https://lore.kernel.org/r/20200103212021.2881-2-logang@deltatee.com


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