platform/chrome: cros_ec_typec: Set partner num_altmodes

Set the number of altmodes available for a registered partner using the
Type C connector class framework routine.

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
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/20201116201150.2919178-10-pmalani@chromium.org