Blackfin: SMP: kgdb: apply anomaly 257 work around

When run kgdb testing, it looks like coreb hangs in single step or trap
exception without handling anomaly 05000257 properly on bf561 v0.5.  But,
the anomaly list says it apply to bf561 v0.4 and bellow.  Apply its work
around to 0.5 temporarily until the behavior and the root cause can be
confirmed by the hardware team.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>