dmaengine: imx-sdma: implement channel termination via worker

The dmaengine documentation states that device_terminate_all may be
asynchronous and need not wait for the active transfers to stop.

This allows us to move most of the functionality currently implemented
in the sdma channel termination function to run in a worker, outside
of any atomic context. Moving this out of atomic context has two
benefits: we can now sleep while waiting for the channel to terminate,
instead of busy waiting and the freeing of the dma descriptors happens
with IRQs enabled, getting rid of a warning in the dma mapping code.

As the termination is now async, we need to implement the
device_synchronize dma engine function which simply waits for the
worker to finish its execution.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>