The XDMAC DMA controller uses a concept of views to be able to handle
descriptors of different sizes.
So far, only the views 1 and 2 were handled by the driver. Unfortunately, we
need some of the configuration fields found in the view 3 in order to support
memset and interleaved transfers.
Add the definition for the view 3 registers, and the needed code to handle view
3 descriptors.