platform/chrome: cros_ec: Put docs with the code

To avoid doc rot, put function documentations with code, not header.
Use kernel-doc style comments for exported functions.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>