Input: atmel_mxt_ts - add support for Google Pixel 2

This change allows atmel_mxt_ts to bind to ACPI-enumerated devices in
Google Pixel 2 (2015).

While newer version of ACPI standard allow use of device-tree-like
properties in device descriptions, the version of ACPI implemented in
Google BIOS does not support them, and we have to resort to DMI data to
specify exact characteristics of the devices (touchpad vs. touchscreen,
GPIO to button mapping, etc).

Pixel 1 continues to use i2c devices and platform data created by
chromeos-laptop driver, since ACPI does not enumerate them.

Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>