Blackfin: fix dma-mapping build errors

The recent deprecation of dma_sync_{sg,single} ironically broke Blackfin
systems.  This is because we don't define dma_sync_sg_for_cpu at all, so
until the DMA asm-generic conversion/cleanup is done after the next
release, simply stub out the dma_sync_sg_for_{cpu,device} functions.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>