[POWERPC] Don't enable cell IOMMU fixed mapping if there are no dma-ranges

In order for the cell IOMMU fixed mapping to work we need "dma-ranges"
properties in the device tree. If there are none then there's no point
enabling the fixed mapping support.

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