powerpc: Only do dynamic DMA zone limits on platforms that need it

Scott's patch 1c98025c

 "Dynamic DMA zone limits" changed
dma_direct_alloc_coherent() to start using dev->coherent_dma_mask.

That seems fair enough, but it exposes the fact that some of the drivers
we care about on IBM platforms aren't setting the coherent mask.

The proper fix is to have drivers set the coherent mask and also have
the platform code honor it.

For now, just restrict the dynamic DMA zone limits to the platforms that
need it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Scott Wood <scottwood@freescale.com>