gpio: uapi: fix kernel-doc warnings

gpio: uapi: fix kernel-doc warnings

Fix kernel-doc warnings, specifically gpioline_info_changed.padding is
not documented and 'GPIO event types' describes defines, which are not
documented by kernel-doc.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20201005070329.21055-2-warthog618@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>