mfd: Add BTN_TOUCH event to ucb1x00-ts

Add BTN_TOUCH event reporting to ucb1x00_ts touchscreen driver.
This will make this touchscreen driver behave consistently wrt.
BTN_TOUCH.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>