Documentation: move DMA-mapping.txt to Doc/PCI/

Move DMA-mapping.txt to Documentation/PCI/.

DMA-mapping.txt was supposed to be moved from Documentation/ to
Documentation/PCI/.  The 00-INDEX files in those two directories
were updated, along with a few other text files, but the file
itself somehow escaped being moved, so move it and update more
text files and source files with its new location.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
cc:	Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>