Add dmaengine driver for X1830.

[周琰杰 (Zhou Yanjie) <zhouyanjie@xxxxxxxxxxxxxx>]

1.Modify the documentation description to make it more relevant.
2.Add the dmaengine bindings for the X1830 SoC from Ingenic.
3.Add support for probing the dma-jz4780 driver on the
  X1830 SoC from Ingenic.
Notice:
The X1830's dma controller is very similar to the X1000, the
difference is that the X1830 has 32 dma channels and the
X1000 has only 8.