There is one but it is not documented. The commit adding support for the device mentions: Serial console: internal 4-pin header, 115200 8n1. So you have to open it up and try to figure out which pin is which. If you do please add it to the ToH.
So, you mean usb-in-router to usb-in-pc?
I don't think that Linux really support usb as the console. It it already a sophisticated device, not a low-level tty...