Input: bcm5974 - add BTN_TOUCH event for mousedev benefit

The mousedev driver requires the use of BTN_TOUCH events to process
ABS_X and ABS_Y events properly, which is what is needed for the
bcm5974-based apple computers to have a functional pointer out-of-the-box.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>