microblaze: Export memory_start for modules

memory_start symbol is needed by kernel modules.
Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/mm/init.c

@@ -41,6 +41,7 @@ char *klimit = _end;
  * have available.
  */
 unsigned long memory_start;
+EXPORT_SYMBOL(memory_start);
 unsigned long memory_end; /* due to mm/nommu.c */
 unsigned long memory_size;