Input: bcm5974 - add support for touchpads found in MacBookAir4,2

Added USB device IDs for MacBookAir4,2 trackpad. Device constants were
copied from the MacBookAir3,2 constants. The 4,2 device specification is
reportedly unchanged from the 3,2 predecessor and seems to work well.

Signed-off-by: Joshua V Dillon <jvdillon@gmail.com>
Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>