ARM: at91: Export at91_ramc_base

After commit f363c407 (ARM: at91: make sdram/ddr register base soc independent)
building at91_cf as a module fails with:
  ERROR: "at91_ramc_base" [drivers/pcmcia/at91_cf.ko] undefined!
  make[1]: *** [__modpost] Error 1
  make: *** [modules] Error 2

Export at91_ramc_base symbol to allow drivers using at91_ramc_*
functions to be built as modules again.

Signed-off-by: Joachim Eastwood <joachim.eastwood@jotron.com>
[nicolas.ferre@atmel.com: modify slightly commit message]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>