Documentation: gpio: Move introduction to driver-api

Move gpio/intro.txt to driver-api/gpio/intro.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>
3 files changed