dmaengine: Add Arm DMA-350 driver

Add an initial driver for the Arm Corelink DMA-350 controller, to
support basic mem-to-mem async_tx. The design here leaves room for more
fun things like peripheral support and scatter-gather chaining to come
in future.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
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v2:
 - Fix build warnings
 - Limit retries for reading live residue