nios2: add ioremap_nocache declaration before include asm-generic/io.h.

A commit for the nds32 architecture bootstrap("asm-generic/io.h:
move ioremap_nocache/ioremap_uc/ioremap_wc/ioremap_wt out of ifndef
CONFIG_MMU") will move the ioremap_nocache out of the CONFIG_MMU ifdef.
This means that in order to suppress re-definition errors we need to
setup #define's before importing asm-generic/io.h.

Signed-off-by: Greentime Hu <greentime@andestech.com>
Reviewed-by: Tobias Klauser <tklauser@distanz.ch>

arch/nios2/include/asm/io.h

@@ -45,6 +45,7 @@ static inline void iounmap(void __iomem *addr)
 	__iounmap(addr);
 }
 
+#define ioremap_nocache ioremap_nocache
 #define ioremap_wc ioremap_nocache
 #define ioremap_wt ioremap_nocache