This patch implements dma_get_required_mask() which may be used by drivers
to probe the optimal DMA descriptor type they should be implementing on the
platform.

I've also tested it this time with the sym_2 driver...making it chose the
correct descriptors for the platform.  (although I don't have a 64 bit
platform with >4GB memory, so I only confirmed it selects the 32 bit
descriptors all the time...)
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>