dmaengine: xdmac: Add scatter gathered memset support

The XDMAC also supports memset operations over discontiguous areas. Add the
necessary logic to support this.

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>