pci: Add a generic, weakly-linked pcibios_fixup_bus

Multiple architectures define this as an empty function, and I'm adding
another one as part of the RISC-V port.  This adds a __weak version of
pci_fixup_bios and deletes 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 export this, so I
just dropped it.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>