PCI: Assign resources on a per-bus basis

Previously, we did resource assignment globally.  This patch splits up
pci_assign_unassigned_resources() so assignment is done for each root bus
in turn.  We check each root bus individually to see whether it needs any
reassignment, and if it does, we assign resources for just that bus.

[bhelgaas: changelog]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>