Strange.  sbus is a child of the root node.  There's no iommu node.

not so strange.  an SS2 has:

bootpath: /sbus0/esp@0,800000/sd@0,0
mainbus0 (root): SUNW,Sun 4/75
[ .. ]
sbus0 at mainbus0 ioaddr 0xf8000000: clock = 20 MHz
   
   How shall I attach this thing???

perhaps you will need to invent a new bus space backend for it, but
you should follow what the code does for sun4c i guess.. i haven't
looked at IIep docs myself.