powerpc/pci: Move PHB discovery for PCI_DN using platforms

powerpc/pci: Move PHB discovery for PCI_DN using platforms

Make powernv, pseries, powermac and maple use ppc_mc.discover_phbs.
These platforms need to be done together because they all depend on
pci_dn's being created from the DT. The pci_dn contains a pointer to
the relevant pci_controller so they need to be created after the
pci_controller structures are available, but before PCI devices are
scanned. Currently this ordering is provided by initcalls and the
sequence is:

  1. PHBs are discovered (setup_arch) (early boot, pre-initcalls)
  2. pci_dn are created from the unflattended DT (core initcall)
  3. PHBs are scanned pcibios_init() (subsys initcall)

The new ppc_md.discover_phbs() function is also a core_initcall so we
can't guarantee ordering between the creation of pci_controllers and
the creation of pci_dn's which require a pci_controller. We could use
the postcore, or core_sync initcall levels, but it's cleaner to just
move the pci_dn setup into the per-PHB inits which occur inside of
.discover_phb() for these platforms. This brings the boot-time path in
line with the PHB hotplug path that is used for pseries DLPAR
operations too.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
[mpe: Squash powermac & maple in to avoid breakage those platforms,
      convert memblock allocs to use kmalloc to avoid warnings]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201103043523.916109-2-oohall@gmail.com