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

Most probe functions do nothing else than checking whether
the machine is compatible to a given string.

Define that string 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.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/6cb9865d916231c38401ba34ad1a98c249fae135.1676711562.git.christophe.leroy@csgroup.eu