PCI: Add bridge window selection functions

Various places in the PCI core code independently decide into which bridge
window a child resource should be placed. It is hard to see whether these
decisions always end up in agreement, especially in the corner cases, and
in some places it requires complex logic to pass multiple resource types
and/or bridge windows around.

Add pbus_select_window() and pbus_select_window_for_type() for cases where
the former cannot be used so that eventually the same helper can be used to
select the bridge window everywhere. Using the same function ensures the
selected bridge window remains always the same and it can be easily
recalculated in-situ allowing simplifying the interfaces between internal
functions in upcoming changes.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250829131113.36754-13-ilpo.jarvinen@linux.intel.com