There are several DART variants; print some more details such that we can

There are several DART variants; print some more details such that we can
distinguish between them.  Pay attention to the apple,dma-range property
that tells us the desired DVA window.  Add support for a new BUS_DMA_FIXED
that allows use of bus_dmamap_load_raw(9) to map things at a pre-determined
DVA.  This last change is needed for the upcoming Apple KMS driver.
Hopefully that is the only driver that will need this, so don't attempt to
turn this into an MI feature.

ok patrick@