Blackfin arch: panic when running on a chip rev below what we are compiled for

If we are running on a chip revision below what we are compiled for,
there will be missing anomaly workarounds, and a panic is inevitable. Do
is sooner, rather than later, so people don't look for bugs that already
have workarounds (that they turned off).

Signed-off-by: Robin Getz <rgetz@blackfin.uclinux.org>
Signed-off-by: Bryan Wu <cooloney@kernel.org>