dmaengine: xdmac: Handle descriptor's view 3 registers

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.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>