dmaengine: axi_dmac: Add synchronization support

Implement the new device_synchronize() callback to allow proper
synchronization when stopping a channel. Since the driver already makes
sure that no new complete callbacks are scheduled after the
device_terminate_all() callback has been called, all left to do in the
device_synchronize() callback is to wait for all currently running complete
callbacks to finish.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>