Exercise the DETECT mechanism of the GPIO peripheral.

Signed-off-by: Chris Laplante <chris@laplante.io>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230728160324.1159090-7-chris@laplante.io
[PMM: fixed coding style nits]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>