Enable USB_XHCI_DBGCAP for debugging and Superspeed serial connections
Starting with Linux 4.13 debugging with a USB 3.0 Host-to-Host debugging cable is supported. While CONFIG_EARLY_PRINTK_USB_XDBC runs a serial logging console via USB 3.0 USB_XHCI_DBGCAP added in Linux 4.16 creates a Superspeed serial connection between both hosts.
I suggest to enable USB_XHCI_DBGCAP (=Y) in kernels beginning with 4.16 to support Superspeed serial connections.