x86/PCI: Allocate resources on a per-bus basis for hot-adding root buses

Previously pcibios_allocate_resources() allocated resources at boot-time
for all PCI devices using for_each_pci_dev().  This patch changes
pcibios_allocate_resources() so we can specify a bus, so we can do
similar allocation when hot-adding a root bus.

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