nios2: add definition of ioremap_wc to io.h

Fix

drivers/gpu/drm/drm_bufs.c: In function 'drm_addmap_core':
drivers/gpu/drm/drm_bufs.c:213:5:
			error: implicit declaration of function 'ioremap_wc'

seen when building nios2:allmodconfig.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Ley Foon Tan <lftan@altera.com>