ttm: Introduce a placeholder for DMA (bus) addresses.

This is right now limited to only non-pool constructs.

[v2: Fixed indentation issues, add review-by tag]

Reviewed-by: Thomas Hellstrom <thomas@shipmail.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Ian Campbell <ian.campbell@citrix.com>