Input: bcm5974 - add MacBookAir4,1 trackpad support

This patch adds trackpad support for the MacBookAir4,1, released in July 2011.
It is very similar to the MacBookAir4,2 patch submitted by Joshua Dillon and
Chase Douglas.

Signed-off-by: Pieter-Augustijn Van Malleghem <p-a@scarlet.be>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>