platform/chrome: cros_ec_typec: Register plug altmodes

Modify the altmode registration (and unregistration) code so that it
can be used by both partners and plugs.

Then, add code to register plug altmodes using the newly parameterized
function. Also set the number of alternate modes for the plug using the
associated Type C connector class function
typec_plug_set_num_altmodes().

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/20201116201150.2919178-12-pmalani@chromium.org