Input: ads7846 - allow specifying irq trigger type in platform data

On some platforms, for example with GPIO interrupts on mpc5121,
it is not possible to configure falling edge interrupts.

Specifying irq trigger type in platform data structure
allows using ads7846 driver on such platforms.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>