Platform: x86: chromeos_laptop - Add Pixel Touchscreen

Instantiate the atmel mxt1664s touchscreen on this system.
The touchscreen may appear at two possible addresses:
0x4a in operational mode.
0x26 in bootloader mode.

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