Hardware abstractions: bus_dma

Used to abstract the process of letting the device talk
to memory while the CPU does something else.

bus_dmamap_t:
Describes DMA mapping
Array of segments (dm_segs)
Number of segments (dm_nsegs)

bus_dma_segment_t
Single DMA segment

Create the mapping
Allocate memory
Load data into memory
Flush DMA cache
Do DMA transfer
Flush DMA cache
Unload data
Free memory