tools: gpio: add debounce support to gpio-event-mon

Add support for debouncing monitored lines to gpio-event-mon.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>