nios2: put setup.h in uapi

This header file is exported, but from a userland pov, it's just a wrapper
to asm-generic/setup.h.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>