drivers: dp: swdp_bitbang: power optimization

This patch changes GPIO initialization
to be in PORT_OFF state by default.
Also, if no transceiver is attached to the signals,
the GPIOs are configured to be disconnected to preserve power.

I tested this on a prototype board where we are going to have
a debugger in a low-power context.

Signed-off-by: Maximilian Deubel <maximilian.deubel@nordicsemi.no>