x86/PCI: Factor out pcibios_allocate_dev_rom_resource()

Factor pcibios_allocate_rom_resources() and
pcibios_allocate_dev_rom_resource() out of pcibios_assign_resources().
This will allow us to allocate ROM resources for hot-added root buses.

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