ARM: at91: Export at91_st_base

After commit 5e9cf5e1 (ARM: at91: make ST (System Timer) soc independent)
building at91rm9200_wdt as a module fails with following message
  ERROR: "at91_st_base" [drivers/watchdog/at91rm9200_wdt.ko] undefined!
  make[1]: *** [__modpost] Error 1
  make: *** [modules] Error 2

Export symbol to allow wdt driver to be built as a module again.
Signed-off-by: Joachim Eastwood <joachim.eastwood@jotron.com>
[nicolas.ferre@atmel.com: use EXPORT_SYMBOL_GPL()]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/mach-at91/at91rm9200_time.c

@@ -23,6 +23,7 @@
 #include <linux/interrupt.h>
 #include <linux/irq.h>
 #include <linux/clockchips.h>
+#include <linux/export.h>
 
 #include <asm/mach/time.h>
 
@@ -176,6 +177,7 @@ static struct clock_event_device clkevt = {
 };
 
 void __iomem *at91_st_base;
+EXPORT_SYMBOL_GPL(at91_st_base);
 
 void __init at91rm9200_ioremap_st(u32 addr)
 {