Blackfin: support smaller uncached DMA chunks for memory constrained systems

When working with 8 meg systems, forcing a 1 meg DMA chunk heavily cuts
into the available resources.  So support smaller chunks to better cover
needs for these systems.

Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>