mfd: cros_ec_dev: Add CEC sub-device registration

The EC can expose a CEC bus, thus add the cros-ec-cec MFD sub-device
when the CEC feature bit is present.

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