sparc32: Make IOMMU and IO-UNIT init work with device nodes.

And stick the iommu archdata pointer into the generic OF device tree
of_device struct as well.

We still have to pass the sbus_bus object down into the routines so
that the SBUS bus objects get the iommu cookies set properly.  After
drivers get converted to being pure OF drivers, that can go away.

Signed-off-by: David S. Miller <davem@davemloft.net>