Input: atmel_mxt_ts - derive phys from i2c client adapter

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: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>