Blackfin: SMP: flush CoreB cache when shutting down

When CoreB wakes up, it needs to read variables that CoreA might have
modified, and might be in CoreB's cache.  So kill CoreB's cache before
going to sleep so that when we wake up, we are in a coherent state.

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