PCI: Cleanup per-arch list of object files

setup-bus.o is now included unconditionally as of commit 7dc30303
("PCI: Always build setup-bus when PCI is enabled").  Remove it from the
per-arch list of object files.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>