Add a pcib_attach_child() method to manage adding the child "pci" device.

This allows the PCI-PCI bridge driver to save a reference to the child
device in its softc.

Note that this required moving the "pci" device creation out of
acpi_pcib_attach(). Instead, acpi_pcib_attach() is renamed to
acpi_pcib_fetch_prt() as it's sole action now is to fetch the PCI
interrupt routing table.

Differential Revision: https://reviews.freebsd.org/D6021