Allwinner A31 DMA Controller

This driver follows the generic DMA bindings defined in dma.txt.

Required properties:

- compatible:	Must be "allwinner,sun6i-a31-dma" or "allwinner,sun8i-a23-dma"
- reg:		Should contain the registers base address and length
- interrupts:	Should contain a reference to the interrupt used by this device
- clocks:	Should contain a reference to the parent AHB clock
- resets:	Should contain a reference to the reset controller asserting
		this device in reset
- #dma-cells :	Should be 1, a single cell holding a line request number

Example:
	dma: dma-controller@01c02000 {
		compatible = "allwinner,sun6i-a31-dma";
		reg = <0x01c02000 0x1000>;
		interrupts = <0 50 4>;
		clocks = <&ahb1_gates 6>;
		resets = <&ahb1_rst 6>;
		#dma-cells = <1>;
	};

Clients:

DMA clients connected to the A31 DMA controller must use the format
described in the dma.txt file, using a two-cell specifier for each
channel: a phandle plus one integer cells.
The two cells in order are:

1. A phandle pointing to the DMA controller.
2. The port ID as specified in the datasheet

Example:
spi2: spi@01c6a000 {
	compatible = "allwinner,sun6i-a31-spi";
	reg = <0x01c6a000 0x1000>;
	interrupts = <0 67 4>;
	clocks = <&ahb1_gates 22>, <&spi2_clk>;
	clock-names = "ahb", "mod";
	dmas = <&dma 25>, <&dma 25>;
	dma-names = "rx", "tx";
	resets = <&ahb1_rst 22>;
};