MFC 340022: Add support for port unit wiring to cxgbe(4).
- Add a bus_child_location_str method to the nexus drivers that prints out 'port=N' as the location string exported via devinfo and the '%location' sysctl node.
- We can't use a bus_hint_device_unit to wire the unit numbers of devices with a fixed devclass as the device gets assigned a unit in make_device() before the device creator can set softc, etc. Instead, when adding a child device, use a helper function much like a bus_hint_device_unit method to look for wiring hints or to return -1 to let the system choose a unit number. This function requires an "at" hint for the port pointing to the nexus device and a "port" hint listing the port number. For example:
hint.cxl.4.at="t5nex0"
hint.cxl.4.port="0"
wires cxl4 to the first port on the t5nex0 adapter.