dmaengine: xilinx: xdma: Prepare the introduction of interleaved DMA transfers

Make generic code generic. As descriptor-filling logic stays the same
regardless of a dmaengine's type of transfer, it is possible to write
the descriptor-filling function in a generic way, so that it can be used
for every single type of transfer preparation callback.

Signed-off-by: Jan Kuliga <jankul@alatek.krakow.pl>
Link: https://lore.kernel.org/r/20231218113943.9099-8-jankul@alatek.krakow.pl
Signed-off-by: Vinod Koul <vkoul@kernel.org>