MIPS: boot: add missing targets for vmlinux.*.its

The build rule of vmlinux.*.its is invoked by $(call if_changed,...)
but it always rebuilds the target needlessly due to missing targets.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Patchwork: https://patchwork.linux-mips.org/patch/19092/
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org