Remove some unused counters.

No need to allocate iomm_table and iobar_table, which means that
iomm_table_initialize is not longer needed.

Use kzalloc where sensible.
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NPaul Mackerras <paulus@samba.org>