x86/PCI: Only enable a 64bit BAR on single-socket AMD Family 15h

When we have a multi-socket system, each CPU core needs the same setup.
Since this is tricky to do in the fixup code, don't enable a 64bit BAR on
multi-socket systems for now.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>