Blackfin: set ARCH_KMALLOC_MINALIGN

Architectures that handle DMA-non-coherent memory need to set
ARCH_KMALLOC_MINALIGN to make sure that kmalloc'ed buffer is DMA-safe:
the buffer doesn't share a cache with the others.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>