powerpc/machdep: Define 'compatibles' property in ppc_md and use it

Most probe functions that do not use the 'compatible' string do
nothing else than checking whether the machine is compatible with
one of the strings in a NULL terminated table of strings.

Define that table of strings in ppc_md structure and check it directly
from probe_machine() instead of using ppc_md.probe() for that.

Keep checking in ppc_md.probe() only for more complex probing.

All .compatible could be replaced with a single element NULL
terminated list but that's not worth the churn. Can be do incrementaly
in follow-up patches.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231214103152.12269-4-mpe@ellerman.id.au