Input: bcm5974 - add quad-finger tapping

The integrated button on the new unibody Macbooks presents a need to
report explicit four-finger actions. Evidently, the finger pressing
the button is also touching the trackpad, so in order to fully support
three-finger actions, the driver must be able to report four-finger
actions. This patch adds a new button, BTN_TOOL_QUADTAP, which
achieves this.

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