Input: ads7846 - introduce .gpio_pendown to get pendown state

The GPIO connected to ADS7846 nPENIRQ signal is usually used to get
the pendown state as well. Introduce a .gpio_pendown, and use this
to decide the pendown state if .get_pendown_state is NULL.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>