[POWERPC] Fix cell IOMMU code to cope with empty dma-ranges and non-PCI devices

The cell IOMMU code to parse the dma-ranges properties, used for the fixed
mapping, was broken in two ways for some devices.

Firstly it didn't cope with empty dma-ranges properties. An empty property
implies no translation so can be safely skipped.

The code also wrongly assumed it would be looking at PCI devices, and hard
coded the number of address and size cells.

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