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>