Documentation: Rename Documentation/DMA-mapping.txt

It seems that Documentation/DMA-mapping.txt was supposed to be renamed
to Documentation/PCI/PCI-DMA-mapping.txt.

Signed-off-by: Kusanagi Kouichi <slash@ac.auone-net.jp>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>