[POWERPC] iommu_free_table doesn't need the device_node

It only needs the iommu_table address.  It also makes use of the node
name to print error messages.  So just pass it the things it needs.
This reduces the places that know about the pci_dn by one.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>