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 <[e-mail]>
Signed-off-by: Mike Frysinger <[e-mail]>

arch/blackfin/include/asm/io.h

···
 #define iowrite16(val, x)		writew(val, x)
 #define iowrite32(val, x)		writel(val, x)
 
-#define mmiowb() wmb()
+/**
+ * I/O write barrier
+ *
+ * Ensure ordering of I/O space writes. This will make sure that writes
+ * following the barrier will arrive after all previous writes.
+ */
+#define mmiowb() do { SSYNC(); wmb(); } while (0)
 
 #define IO_SPACE_LIMIT 0xffffffff