powerpc/pci: Make pcibios_make_OF_bus_map() static

Function pcibios_make_OF_bus_map() is used only in pci_32.c. So make it
static.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220706104308.5390-3-pali@kernel.org