platform/chrome: cros_ec_i2c: switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management. Also fix the module license mismatch and change the
description for a more descriptive phrase.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>