Blackfin: only handle CPLB protection violations when MPU is enabled

We don't need to handle CPLB protection violations unless we are running
with the MPU on.  Fix the entry code to call common trap_c, and remove the
code which is never run.  This allows the traps test suite to run on older
boards with the MPU disabled.

URL: http://blackfin.uclinux.org/gf/tracker/5129
Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>