powerpc/platforms: Use 'compatible' property for simple cases

Use the new 'compatible' property for simple cases.

checkpatch complains about the new compatible being undocumented
but in reality nothing is new so just ignore it for the time
being.

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