PCI: Allow root and child buses to have different pci_ops

PCI host bridges often have different ways to access the root and child
bus config spaces. The host bridge drivers have invented their own
abstractions to handle this. Let's support having different root and
child bus pci_ops so these per driver abstractions can be removed.

Link: https://lore.kernel.org/r/20200821035420.380495-2-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>