bcm5974(4): add Magic Trackpad 2 (USB only) support

The MT2 uses a compact report format, but otherwise is similar in many
ways to the internal trackpads, it even uses the same mode switching
commands.

Reviewed by: wulf
Differential revision: https://reviews.freebsd.org/D34437

(cherry picked from commit ef8397c28e981e0115d435aabcf57cee7ea5f5ee)