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>
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index e9b41b1..6d8352c 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -44,7 +44,7 @@
    uio-howto
    firmware/index
    pinctl
-   gpio
+   gpio/index
    misc_devices
    dmaengine/index
    slimbus