gpio: cdev: go back to storing debounce period in the GPIO descriptor

This effectively reverts commits 9344e34e ("gpiolib: cdev: relocate
debounce_period_us from struct gpio_desc") and d8543cba ("gpiolib:
remove debounce_period_us from struct gpio_desc") and goes back to
storing the debounce period in microseconds in the GPIO descriptor

We're doing it in preparation for notifying the user-space about
in-kernel line config changes.

Reviewed-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20241018-gpio-notify-in-kernel-events-v5-3-c79135e58a1c@linaro.org


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>