ARM: OMAP2+: Fix module build errors with CONFIG_OMAP4_ERRATA_I688

While building modules with randconfig the below errors are observed.

ERROR: "omap_bus_sync" [drivers/watchdog/sp805_wdt.ko] undefined!
ERROR: "omap_bus_sync" [drivers/watchdog/dw_wdt.ko] undefined!
ERROR: "omap_bus_sync" [drivers/virtio/virtio_ring.ko] undefined!
ERROR: "omap_bus_sync" [drivers/video/sm501fb.ko] undefined!
ERROR: "omap_bus_sync" [drivers/usb/mon/usbmon.ko] undefined!
ERROR: "omap_bus_sync" [drivers/usb/host/sl811-hcd.ko] undefined!
ERROR: "omap_bus_sync" [drivers/usb/host/ohci-hcd.ko] undefined!
ERROR: "omap_bus_sync" [drivers/usb/host/isp1760.ko] undefined!
ERROR: "omap_bus_sync" [drivers/usb/host/isp1362-hcd.ko] undefined!
ERROR: "omap_bus_sync" [drivers/usb/host/isp116x-hcd.ko] undefined!
ERROR: "omap_bus_sync" [drivers/usb/core/usbcore.ko] undefined!
ERROR: "omap_bus_sync" [drivers/tty/serial/altera_uart.ko] undefined!
ERROR: "omap_bus_sync" [drivers/tty/serial/altera_jtaguart.ko] undefined!
ERROR: "omap_bus_sync" [drivers/tty/serial/8250/8250_dw.ko] undefined!
ERROR: "omap_bus_sync" [drivers/ssb/ssb.ko] undefined!
ERROR: "omap_bus_sync" [drivers/rtc/rtc-cmos.ko] undefined!
ERROR: "omap_bus_sync" [drivers/rtc/rtc-bq4802.ko] undefined!
ERROR: "omap_bus_sync" [drivers/mtd/nand/tmio_nand.ko] undefined!
ERROR: "omap_bus_sync" [drivers/mtd/nand/omap2.ko] undefined!
Signed-off-by: NR Sricharan <r.sricharan@ti.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/omap4-common.c

@@ -31,6 +31,7 @@
 
 #include "common.h"
 #include "omap4-sar-layout.h"
+#include <linux/export.h>
 
 #ifdef CONFIG_CACHE_L2X0
 static void __iomem *l2cache_base;
@@ -55,6 +56,7 @@ void omap_bus_sync(void)
 		isb();
 	}
 }
+EXPORT_SYMBOL(omap_bus_sync);
 
 /* Steal one page physical memory for barrier implementation */
 int __init omap_barrier_reserve_memblock(void)