iio: cros_ec_sensors: add cros_ec_activity driver

iio: cros_ec_sensors: add cros_ec_activity driver

ChromeOS EC can report activity information derived from the
accelerometer:
- Reports on-body/off-body as a proximity event.
- Reports significant motion as an activity event.

This new sensor is a virtual sensor, included only when the EC firmware
is compiled with the appropriate module.

Signed-off-by: Gwendal Grignou <gwendal@google.com>
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://patch.msgid.link/20250604053903.1376465-1-gwendal@google.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>