[POWERPC] PS3: Fix DMA scatter-gather

Add the missing pieces to support DMA scatter-gather on the PS3 system bus.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>