[POWERPC] Cell fixup DMA offset for new southbridge

This patch makes the Cell DMA code work on both the Spider and the Axon
south bridges by turning cell_dma_valid into a variable instead of a
constant. This is a temporary patch until we have full iommu support.

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