Blackfin: allow NMI watchdog to be used w/RETN as a scratch reg

NMIs are not safe to return from because many anomaly workarounds are
implemented by disabling interrupts.  The NMI obviously violates this
assumption.  Since the NMI watchdog never returns, we don't have to
worry about it clobbering RETN when it is being used as a scratch register
with the exception stack.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>