powerpc/83xx: handle machine check caused by watchdog timer

When the watchdog timer is set in interrupt mode, it causes a
machine check when it times out. The purpose of this mode is to
ease debugging, not to crash the kernel and reboot the machine.

This patch implements a special handling for that, in order to not
crash the kernel if the watchdog times out while in interrupt or
within the idle task.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[scottwood: added missing #include]
Signed-off-by: Scott Wood <oss@buserror.net>