Implement nRF51 DETECT signal in the GPIO peripheral.

The reference manual makes mention of a per-pin DETECT signal, but these
are not exposed to the user. See https://devzone.nordicsemi.com/f/nordic-q-a/39858/gpio-per-pin-detect-signal-available


for more information. Currently, I don't see a reason to model these.

Signed-off-by: Chris Laplante <chris@laplante.io>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230728160324.1159090-2-chris@laplante.io
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>