Fully unify all of the DMA ops so that subordinate bus types to
the DMA operation providers (such as ebus, isa, of_device) can
work transparently.

Basically, we just make sure that for every system device we
create, the dev->archdata 'iommu' and 'stc' fields are filled
in.

Then we have two platform variants of the DMA ops, one for SUN4U which
actually programs the real hardware, and one for SUN4V which makes
hypervisor calls.

This also fixes the crashes in parport_pc on sparc64, reported by
Meelis Roos.
Signed-off-by: David S. Miller <davem@davemloft.net>