powerpc/setup: Refactor/untangle panic notifiers

The panic notifiers infrastructure is a bit limited in the scope of
the callbacks - basically every kind of functionality is dropped
in a list that runs in the same point during the kernel panic path.
This is not really on par with the complexities and particularities
of architecture / hypervisors' needs, and a refactor is ongoing.

As part of this refactor, it was observed that powerpc has 2 notifiers,
with mixed goals: one is just a KASLR offset dumper, whereas the other
aims to hard-disable IRQs (necessary on panic path), warn firmware of
the panic event (fadump) and run low-level platform-specific machinery
that might stop kernel execution and never come back.

Clearly, the 2nd notifier has opposed goals: disable IRQs / fadump
should run earlier while low-level platform actions should
run late since it might not even return. Hence, this patch decouples
the notifiers splitting them in three:

- First one is responsible for hard-disable IRQs and fadump,
should run early;

- The kernel KASLR offset dumper is really an informative notifier,
harmless and may run at any moment in the panic path;

- The last notifier should run last, since it aims to perform
low-level actions for specific platforms, and might never return.
It is also only registered for 2 platforms, pseries and ps3.

The patch better documents the notifiers and clears the code too,
also removing a useless header.

Currently no functionality change should be observed, but after
the planned panic refactor we should expect more panic reliability
with this patch.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220427224924.592546-9-gpiccoli@igalia.com