MIPS: zboot: Remove copied source files on clean

The copied source files must be added to the extra-y list to have them
removed on clean.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Alex Smith <alex.smith@imgtec.com>
Cc: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12233/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>