[PATCH] powerpc iommu: minor cleanup

A couple of minor renames:

* The iommu_table is no longer a part of the device node structure,
  so devnode_table is misleading
* Rename struct device *-variables to hwdev

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>