PCI: Introduce pci_real_dma_dev()

The current DMA alias implementation requires the aliased device be on the
same PCI bus as the requester ID.  Add an arch-specific mechanism to point
to another PCI device when doing mapping and PCI DMA alias search.  The
default case returns the actual device.

Link: https://lore.kernel.org/r/1579613871-301529-4-git-send-email-jonathan.derrick@intel.com


Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Christoph Hellwig <hch@lst.de>