Input: gpio_keys - convert struct descriptions to kernel-doc

This patch converts descriptions of the structures defined in
linux/gpio_keys.h to follow kernel-doc format.

There is no functional change.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>