dt-bindings: dmaengine: Convert Allwinner A31 and A64 DMA to a schema

The newer Allwinner SoCs have a DMA controller supported in Linux, with a
matching Device Tree binding.

Now that we have the DT validation in place, let's convert the device tree
bindings for that controller over to a YAML schemas.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://lore.kernel.org/r/20190720092607.31095-3-maxime.ripard@bootlin.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>