dmaengine: xilinx_vdma: Improve SG engine handling

The current driver allows user to queue up multiple segments
on to a single transaction descriptor. User will submit this single desc
and in the issue_pending() we decode multiple segments and submit to SG HW engine.
We free up the allocated_desc when it is submitted to the HW.

Existing code prevents the user to prepare multiple trasactions at same time as
we are overwrite with the allocated_desc.

The best utilization of HW SG engine would happen if we collate the pending
list when we start dma this patch updates the same.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>