MIPS: init: move externs to header file

This commit fixes the following checkpatch warnings:

    WARNING: externs should be avoided in .c files

    This is a warning for placing declarations in a ".c" file.
    This fix removes the declaration in ".c" and adds it to
    the common header file.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>