udbc: Add usb debug host mode driver

xhci offers a debugging interface which uses a special usb 3 cable with the D+,
D- and VBUS pairs disconnected. This interface allows a target device to
configure its xhci controller as a debugging channel which can then be used to
provide a serial link between the target and a debug host.

This change extracts the udbc host mode driver from hrs@'s xhci debug
implementation.

Reviewed by: bcr (man page)
MFC after: Before 15-ALPHA3 builds
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51299