[POWERPC] Make IOMMU code safe for > 132 GB of memory

Currently the IOMMU code allocates one page for the segment table, that
isn't safe if we have more than 132 GB of RAM.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>