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

A future 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>

arch/xtensa/include/asm/io.h

@@ -52,6 +52,7 @@ static inline void __iomem *ioremap_cache(unsigned long offset,
 		return xtensa_ioremap_cache(offset, size);
 }
 #define ioremap_cache ioremap_cache
+#define ioremap_nocache ioremap_nocache
 
 #define ioremap_wc ioremap_nocache
 #define ioremap_wt ioremap_nocache