Input: ads7846 - enable pendown GPIO debounce time setting

Some platforms need the pendown GPIO debounce time setting programmed.
Since the pendown GPIO is handled by the driver, the debounce time
should also be handled along with the pendown GPIO request.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>