[MIPS] BCM1480: Export zbbus_mhz.

Symbol is required by the ZBus profiler.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/sibyte/bcm1480/setup.c

@@ -35,6 +35,7 @@ unsigned int soc_type;
 EXPORT_SYMBOL(soc_type);
 unsigned int periph_rev;
 unsigned int zbbus_mhz;
+EXPORT_SYMBOL(zbbus_mhz);
 
 static unsigned int part_type;