driver: swdp_bitbang: rework pin configurations

Move low-level GPIO functions to a separate file and use GPIO driver
API if low-level GPIO support is not available for the platform.
Allows alternative pin configuration using only two pins, clk and dio.
Improve binding description.

Signed-off-by: Maximilian Deubel <maximilian.deubel@nordicsemi.no>
Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>