Platform: x86: chromeos_laptop - Add support for probing devices

This will allow support for devices that may appear at more than
one i2c address at boot time. The specific example is the atmel_mxt touch
devices, which may appear at a different address if it comes up
in bootloader mode.

Signed-off-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>