Removed reference to non-existing file Documentation/PCI/PCI-DMA-mapping.txt

File Documentation/PCI/PCI-DMA-mapping.txt does not exist.
 Documentation/DMA-mapping.txt contains DMA Mapping details

Signed-off-by: vibi sreenivasan <vibi_sreenivasan@cms.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>