Input: bcm5974 - set BUTTONPAD property

Some bcm5974 trackpads have a physical button beneath the physical surface.
This patch sets the property bit so user space applications can detect the
trackpad type and act accordingly.

Signed-off-by: Jussi Pakkanen <jussi.pakkanen@canonical.com>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>