Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Clean up some magic numbers in the DBDMA code by replacing them with
appropriately defined constants.

Suggested by: gnn

Change the DBDMA API to allow DBDMA registers in a subregion of a resource. This is necessary to allow future support of DMA for the various Apple on-board ATA controllers.

MFC after: 1 week

Add support for Apple's Descriptor-Based DMA (DBDMA) engine. The DMA
engine is usful to various existing drivers, such as ata(4) and scc(4),
and is used bhy the soon to be added bm(4).

Submitted by: Nathan Whitehorn