mfd: cros-ec: Introduce CEC commands and events definitions.

The EC can expose a CEC bus, this patch adds the CEC related definitions
needed by the cros-ec-cec driver.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>