usb: typec: Add class for plug alt mode device

Add the Type C class for plug alternate mode devices which are being
registered by the Type C connector class. This ensures that udev events
get generated when the plug alt modes are registered.

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Benson Leung <bleung@chromium.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Prashant Malani <pmalani@chromium.org>
Link: https://lore.kernel.org/r/20201210211653.879044-1-pmalani@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>