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>