MIPS: Remove all generated vmlinuz* files on "make clean"

[Ralf: I changed the patch to explicitly list all files to be deleted out
of paranoia.]

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/1590/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>