Save quite a bit of space by removing the existence of PTEDs. The

Save quite a bit of space by removing the existence of PTEDs.  The
dynamics of SMMU are a bit different to regular MMU usage, as we do
not need P->V lists or ref/mod emulation (with page access upgrade).
While in the future we might want to save cacheability modes, it is
not necessary right now.  Our PTED construct, which holds that kind
of information, is not needed.  With these gone, we save around 93%
of smmu(4)'s previous memory overhead.

Discussed with drahn@ kettenis@