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 <gregory.estrade@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Message-id: 1457467526-8840-6-git-send-email-Andrew.Baumann@microsoft.com
[AB: implement 2D mode, cleanup/refactoring for upstream submission]
Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
