Documentation: gpio: Move drivers-on-gpio.txt to driver-api

Move gpio/drivers-on-gpio.txt to driver-api/gpio/drivers-on-gpio.rst and
make sure it builds cleanly as ReST.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>