Input: ad7877 - implement EV_KEY:BTN_TOUCH reporting

Some input users such as Android or X 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.

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