[MIPS] Fix marge error due to conflict in arch/mips/kernel/head.S

__INIT directive just before kernel_entry was ignored for most platforms.
This patch fixes it and get rid of this warning:

WARNING: vmlinux.o(.text+0x478): Section mismatch: reference to .init.text:start_kernel (between '_stext' and 'run_init_process')

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>