BACKPORT: UPSTREAM: 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>
(cherry picked from commit c430955d)

Conflict:
  cros_ec_activity.c: write_event_config() `state` is a int in 6.12, not a bool.

Bug: 381480028
Test: On crota360, check cros-ec-activity is loaded, a proximity event
is reported.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Change-Id: Ibdebfe2b19273fcb39a78b8afa88af317763e700