target-ppc: Disentangle hash mmu helper functions

The newly separated paths for hash mmus rely on several helper functions
which are still shared with 32-bit hash mmus: pp_check(), check_prot() and
pte_update_flags().  While these don't have ugly ifdefs on the mmu type,
they're not very well thought out, so sharing them impedes cleaning up the
hash mmu paths.  For now, put near-duplicate versions into mmu-hash64.c and
mmu-hash32.c, leaving the old version in mmu_helper.c for 6xx software
loaded tlb implementations.  The hash 32 and software loaded
implementations are simplfied slightly, using the fact that no 32-bit CPUs
implement the 3rd page protection bit.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
