Input: ad7879 - report EV_KEY/BTN_TOUCH events

Some input events users such as Android require BTN_TOUCH events.
Implement EV_KEY/BTN_TOUCH and make sure that the release event
is not erroneous scheduled without a preceding valid touch.
Avoid duplicated BTN_TOUCH events, even though input core filters
them.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>