PCI: Add defines for bridge window indexing

PCI: Add defines for bridge window indexing

include/linux/pci.h provides PCI_BRIDGE_{IO,MEM,PREF_MEM}_WINDOW defines,
however, they're based on the resource array indexing in the pci_dev
struct. The struct pci_bus also has pointers to those same resources but
they start from zeroth index.

Add PCI_BUS_BRIDGE_{IO,MEM,PREF_MEM}_WINDOW defines to get rid of literal
indexing.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250829131113.36754-12-ilpo.jarvinen@linux.intel.com