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>

arch/nios2/include/asm/io.h

@@ -45,6 +45,8 @@ static inline void iounmap(void __iomem *addr)
 	__iounmap(addr);
 }
 
+#define ioremap_wc ioremap_nocache
+
 /* Pages to physical address... */
 #define page_to_phys(page)	virt_to_phys(page_to_virt(page))
 #define page_to_bus(page)	page_to_virt(page)