- merge IOMMU DVMA code from sbus/psycho into iommu.c. this code was
identical and a few minor inconsistencies had crept in. this way
keeps them all in sync.
- with this code gone from psycho, merge the psycho.c and psycho_bus.c
files. same with ebus/ebus_bus.c. delete the _bus.c files.
- add a _ds_boundary member to the dma segment structure, so that later
dma mappings can find this value.
- set _ds_boundary in machdep.c:_bus_dmamem_alloc().
- kill much dead code.
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