platform/chrome: cros_ec_lpc: switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management. Also remove the license boiler-plate and redundant driver
description.

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