leon3: use qdev gpio facilities for the PIL

leon3: use qdev gpio facilities for the PIL

As Peter Maydell once said:
"Creating a gpio pin on some object that isn't yourself
looks a bit odd, but all this leon3 code is modifying
the CPU object from the outside anyway. Someday we might
tidy it up, but not today."

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>