[POWERPC] Add an optional offset to direct DMA on 64 bits

This patch adds an optional global offset that can be added to DMA addresses
when using the direct DMA operations.

That brings it a step closer to the 32 bits direct DMA operations, and makes
it useable on Cell when the MMU is disabled and we are using a spider
southbridge.

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