ARM: at91: Export at91_pmc_base

After commit b5514952 (ARM: at91/PMC: make register base soc independent)
building atmel_usba_udc as a module fails with following message
  ERROR: "at91_pmc_base" [drivers/usb/gadget/atmel_usba_udc.ko] undefined!
  make[1]: *** [__modpost] Error 1
  make: *** [modules] Error 2

Export symbol to allow driver to be built as a module again.

Signed-off-by: Joachim Eastwood <joachim.eastwood@jotron.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>