dma-mapping: frv: remove the obsolete and unnecessary DMA API comments

pci_dma_sync_single was obsoleted long ago.

All the comments are generic, not architecture specific, simply describes
some of the DMA-API (and frv has the same comments in three files).
Documentation/DMA-API.txt have more detailed descriptions.

This removes the above obsolete and unnecessary DMA API comments.  Let's
describe the DMA API in only Documentation/DMA-API.txt.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: David Howells <dhowells@redhat.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>