powerpc/eeh: Move EEH initialisation to an arch initcall

The initialisation of EEH mostly happens in a core_initcall_sync initcall,
followed by registering a bus notifier later on in an arch_initcall.
Anything involving initcall dependecies is mostly incomprehensible unless
you've spent a while staring at code so here's the full sequence:

ppc_md.setup_arch       <-- pci_controllers are created here

...time passes...

core_initcall           <-- pci_dns are created from DT nodes
core_initcall_sync      <-- platforms call eeh_init()
postcore_initcall       <-- PCI bus type is registered
postcore_initcall_sync
arch_initcall           <-- EEH pci_bus notifier registered
subsys_initcall         <-- PHBs are scanned here

There's no real requirement to do the EEH setup at the core_initcall_sync
level. It just needs to be done after pci_dn's are created and before we
start scanning PHBs. Simplify the flow a bit by moving the platform EEH
inititalisation to an arch_initcall so we can fold the bus notifier
registration into eeh_init().

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200918093050.37344-5-oohall@gmail.com