dmaengine: sun6i: Add support for Allwinner A23 (sun8i) variant
The A23 SoC has the same dma engine as the A31 (sun6i), with a
reduced amount of endpoints and physical channels. Add the proper
config data and compatible string to support it.
A slight difference in sun8i is an undocumented register needs
to be toggled for dma to function.