platform/chrome: cros_ec_typec: Register SOP' cable plug

In order to register cable alternate modes, we need to first register a
plug object. Use the Type C connector class framework to register a SOP'
plug for this purpose.

Since a cable and plug go hand in hand, we can handle the registration
and removal together.

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Benson Leung <bleung@chromium.org>
Link: https://lore.kernel.org/r/20201116201150.2919178-11-pmalani@chromium.org