Documentation: gpio: Move legacy documentation to driver-api

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

Also move the legacy API reference from index.rst to legacy.rst.

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