docs: driver-api: Add GPIO section

This adds a section about the various parts of the GPIO subsystem to the
driver API documentation. Note that this isn't exhaustive documentation,
but rather focusses on pulling in the kerneldoc from various sources, in
order to improve coverage of kerneldoc processing.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>