Add a parent device field to struct pci_config_window.  The parent is not
saved now, but will be useful to save it in some cases.  For ACPI on ARM64,
it can be used to setup ACPI companion and domain.

Since the parent dev is in struct pci_config_window now, we need not pass
it to the init function as a separate argument.
Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>