platform/chrome: Add support for the acer c720p touchscreen.

Add support for the acer c720p touchscreen.
Tested manually by using the touchscreen on the acer c720p-2664

Based on the following patch by Dave Parker <dparker@chromium.org>:
https://chromium-review.googlesource.com/#/c/167136/Signed-off-by: NMichael Mullin <masmullin@gmail.com>
Reviewed-by: NBenson Leung <bleung@chromium.org>
Signed-off-by: NOlof Johansson <olof@lixom.net>

drivers/platform/chrome/chromeos_laptop.c

@@ -419,6 +419,8 @@ static struct chromeos_laptop acer_ac700 = {
 
 static struct chromeos_laptop acer_c720 = {
 	.i2c_peripherals = {
+		/* Touchscreen. */
+		{ .add = setup_atmel_1664s_ts, I2C_ADAPTER_DESIGNWARE_1 },
 		/* Touchpad. */
 		{ .add = setup_cyapa_tp, I2C_ADAPTER_DESIGNWARE_0 },
 		/* Light Sensor. */