greybus: manifest: check for duplicate CPort descriptors when parsing

Now that connection creation has been separated from interface
initialisation, we should explicitly check for duplicate CPort
descriptors when parsing the manifest.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>