powerpc: Merge pci.h

This involves some minor changes: a few unused functions that the
ppc32 pci.c provides are no longer declared here or exported;
pcibios_assign_all_busses now just refers to the pci_assign_all_buses
variable on both 32-bit and 64-bit; pcibios_scan_all_fns is now
just 0 instead of a function that always returns 0 on 64-bit.

Signed-off-by: Paul Mackerras <paulus@samba.org>