swd_gpio.cpp: emulate freedom board

Signed-off-by: Federico Vaga <federicov@linino.org>
swd_gpio: init clock to low

Signed-off-by: Federico Vaga <federicov@linino.org>
swd_gpio: store data if pointer available

Signed-off-by: Federico Vaga <federicov@linino.org>
swd_gpio: DIO low on turnaround

Signed-off-by: Federico Vaga <federicov@linino.org>
swd_gpio: add debug message in write

Signed-off-by: Federico Vaga <federicov@linino.org>