bcm2835_dma: add emulation of Raspberry Pi DMA controller

bcm2835_dma: add emulation of Raspberry Pi DMA controller

At present, all DMA transfers complete inline (so a looping descriptor
queue will lock up the device). We also do not model pause/abort,
arbitrarion/priority, or debug features.

Signed-off-by: Grégory ESTRADE <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Andrew Baumann <[email protected]>
Message-id: 1457467526[email protected]
[AB: implement 2D mode, cleanup/refactoring for upstream submission]
Signed-off-by: Andrew Baumann <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>