PCI: Add a generic weak pcibios_fixup_bus()

Multiple architectures define this as an empty function, and I'm adding
another one as part of the RISC-V port.  Add a __weak version of
pcibios_fixup_bus() and delete the now-obselete ones in a handful of
ports.

The only functional change should be that microblaze used to export
pcibios_fixup_bus().  None of the other architectures exports this, so I
just dropped it.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

arch/microblaze/pci/pci-common.c

@@ -810,12 +810,6 @@ void pcibios_setup_bus_devices(struct pci_bus *bus)
 	}
 }
 
-void pcibios_fixup_bus(struct pci_bus *bus)
-{
-	/* nothing to do */
-}
-EXPORT_SYMBOL(pcibios_fixup_bus);
-
 /*
  * We need to avoid collisions with `mirrored' VGA ports
  * and other strange ISA hardware, so we always want the