docs: driver-api: Move bt8xxgpio to the gpio directory

Let's declutter Documentation/driver-api a bit.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20191004164059.10397-1-j.neuschaefer@gmx.net


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>