[POWERPC] Make direct DMA use node local allocations

This patch makes dma_alloc_coherent() use node local allocation when
using the direct DMA ops. The node is obtained from the new device
extension. If no such extension is present, the current node is used.

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