PPC: Disable non-440 CPUs for ppcemb target

PPC: Disable non-440 CPUs for ppcemb target

The sole reason we have the ppcemb target is to support MMUs that have
less than the usual 4k possible page size. There are very few of these
chips and I don't want to add additional QA and testing burden to everyone
to ensure that code still works when TARGET_PAGE_SIZE is not 4k.

So this patch disables all CPUs except for MMU_BOOKE capable ones from
the ppcemb target.

Signed-off-by: Alexander Graf <agraf@suse.de>