Blackfin: make sure mmiowb inserts a write barrier with SSYNC

Add SSYNC to our implementation of I/O write barrier to ensure ordering
of I/O space writes.  This will make sure that writes following the
barrier will arrive after all previous writes.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>