Input: ads7846 - add support for the ads7843 touchscreen

The ads7843 support has now become almost trivial since the last
rework.

Signed-off-by: Nicolas Ferre <nicolas.ferre@rfo.atmel.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>