dmaengine: xdmac: Add memset support

The XDMAC supports memset transfers, both over contiguous areas, and over
discontiguous areas through a LLI.

The current memset operation only supports contiguous memset for now, add some
support for it. Scatter-gathered memset will come eventually.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>