This allows userspace to more easily distinguish which bus a particular
atmel_mxt_ts device is attached to.

The resulting phys will be something like:
 i2c-1-0067/input0
Signed-off-by: NDaniel Kurtz <djkurtz@chromium.org>
Signed-off-by: NHenrik Rydberg <rydberg@euromail.se>