unicore32: add ioremap_nocache definition

Bugfix for following error messages:
lib/iomap.c: In function 'pci_iomap':
lib/iomap.c:274: error: implicit declaration of function 'ioremap_nocache'
lib/iomap.c:274: warning: return makes pointer from integer without a cast

Also see commit <f1ecc698>
  it will hide the ioremap_nocache function for systems with an MMU

Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jonas Bonn <jonas@southpole.se>