dmaengine: ste_dma40: add a done queue for completed descriptors

dmaengine: ste_dma40: add a done queue for completed descriptors

This is to keep the active queue for only those transfers which are
actually active in the hardware.  Descriptors will be moved to the done
queue after they are completed in the hardware (interrupt handler) but
before all the cleanup work has been completed (tasklet).

Mostly based on a previous patch by Rabin Vincent.

Cc: Rabin Vincent <rabin.vincent@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>