platform/chrome: Add cros_ec_accel_legacy driver

Add driver to support older EC firmware that only support deprecated
ec command. Rely on ACPI memory map register to access sensor
information.
Present same interface as the regular cros_ec sensor stack:
- one iio device per accelerometer
- use HTML5 axis definition
- use iio abi units
- accept calibration calls, but do nothing
Chrome can use the same code than regular cros_ec sensor stack to
calculate orientation and lid angle.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio-cros-ec

@@ -16,3 +16,13 @@ Description:
                 the motion sensor is placed. For example, in a laptop a motion
                 sensor can be located on the base or on the lid. Current valid
 		values are 'base' and 'lid'.
+
+What:		/sys/bus/iio/devices/iio:deviceX/id
+Date:		Septembre 2017
+KernelVersion:	4.14
+Contact:	linux-iio@vger.kernel.org
+Description:
+		This attribute is exposed by the CrOS EC legacy accelerometer
+		driver and represents the sensor ID as exposed by the EC. This
+		ID is used by the Android sensor service hardware abstraction
+		layer (sensor HAL) through the Android container on ChromeOS.