blackfin enable arbitary speed serial setting

Add the needed definitions to activate arbitary speed support on the blackfin platform.

Signed-off-by: Alan Cox <alan@redhat.com>
Acked-by: Aubrey Li <aubrey.li@analog.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>