Documentation: driver-api: Move gpio.rst to gpio/index.rst

To make space for more files in the GPIO section, create a
Documentation/driver-api/gpio/ directory.
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/index.rst

@@ -44,7 +44,7 @@ available subsections can be seen below.
    uio-howto
    firmware/index
    pinctl
-   gpio
+   gpio/index
    misc_devices
    dmaengine/index
    slimbus