Documentation: rename PCI/PCI-DMA-mapping.txt to DMA-API-HOWTO.txt

This patch renames PCI/PCI-DMA-mapping.txt to DMA-API-HOWTO.txt.

The commit 51e7364e

 "Documentation: rename
PCI-DMA-mapping.txt to DMA-API-HOWTO.txt" was supposed to do this but it
didn't.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>