dmaengine: xilinx: Introduce synchronize() callback

The Xilinx dmaengine driver uses a tasklet to process completed
descriptors and execute their callbacks.

Currently consumers of the DMA channel have to no method of synchronization
against this tasklet when using the Xilinx dmaengine drivers. This can lead
to race conditions when the consumer frees resources that are accessed in
the callback before the tasklet has finished running.

It is not enough to just call dmaengine_terminal_all() since on a
multi-processor system the tasklet can run concurrently to it and might
call the callback after dmaengine_terminate_all() has already finished.

To mitigate this issue implement the synchronize() callback for the driver,
which will wait until the tasklet has finished.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210313125311.4823-1-lars@metafoo.de
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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