class PciChildVisitor

Visits PCI bus nodes (those with `device_type = "pci"`) and wires each of

their children to the PCI device that the PCI bus driver discovers at the

child's bus/device/function (BDF) address.

Unlike the host-controller visitor (//sdk/lib/driver/devicetree/visitors/

drivers/pci), which parses the controller's own resources, this visitor is

controller agnostic: it only needs the standard `device_type = "pci"` marker

and the children's `reg` BDF encoding, so it works for any PCI bus (including

PCI-to-PCI bridges, which also carry `device_type = "pci"`).

For each child it adds a composite node spec parent that accepts the PCI bus

driver's per-device fragment, identified by its PCI topology (BDF). The

child's other devicetree resources (gpios, regulators, clocks, ...) are added

as additional parents by their respective visitors, so the resulting

composite gives the child driver access to its sideband resources alongside

the PCI bus.