Documentation: convert PCI-DMA-mapping.txt to use the generic DMA API

- replace the PCI DMA API (i.e. pci_dma_*) with the generic DMA API.

- make the document more generic (use the PCI specific explanation as
  an example).

[akpm@linux-foundation.org: fix things Randy noticed]
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: "David S. Miller" <davem@davemloft.net>
Reviewed-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>