[POWERPC] Cell iommu support

This patch adds full cell iommu support (and iommu disabled mode).

It implements mapping/unmapping of iommu pages on demand using the
standard powerpc iommu framework.  It also supports running with
iommu disabled for machines with less than 2GB of memory.  (The
default is off in that case, though it can be forced on with the
kernel command line option iommu=force).

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>