Provide dma_map_page() for the NOMMU-mode FRV arch.

Also do some fixing on the comments attached to the various DMA functions for
both MMU and NOMMU mode FRV code.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>