Blackfin: export the last exception cause via debugfs

We have some test code that runs in userspace that exercises the exception
handling of the Blackfin pretty thoroughly.  Part of the validation process
is checking the exact exception triggered, so export the last one seen to
userspace via debugfs when debugging is enabled for the test code to check.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>