Blackfin arch: ISP1761 doesn't work for USB flash disk

ZONE_DMA is required for some drivers subsystem, such as USB/SCSI.

Signed-off-by: Aubrey Li <aubrey.li@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>