decodetree: Allow multiple input files

decodetree: Allow multiple input files

While it would be possible to concatenate input files with make,
passing the original input files to decodetree.py allows us to
generate error messages which allows compilation environments
(read: emacs) to next-error to the correct input file.

Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>