lib/powerpc/setup: comment init dependencies

cpu_init must be called before mem_init because cpu_init determines
the cache-line sizes. Currently io_init has no dependency on
mem_init, but io_init should be allowed to use heap allocation,
and someday it may, so we proactively comment it.

Cc: Laurent Vivier <lvivier@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>