arm64: allwinner: a64: Add devicetree binding for DMA controller

The A64 is register compatible with the H3, but has a different number
of dma channels and request ports.

Attach additional properties to the node to allow future reuse of the
compatible for controllers with different number of channels/requests.

If dma-requests is not specified, the register layout defined maximum
of 32 is used.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>