dt-bindings: dmaengine: add DT binding for UniPhier MIO DMAC

UniPhier Media IO DMA controller

This works as an external DMA engine for SD/eMMC controllers etc.

found in UniPhier LD4, Pro4, sLD8 SoCs.

Required properties:

- compatible: should be "socionext,uniphier-mio-dmac".

- reg: offset and length of the register set for the device.

- interrupts: a list of interrupt specifiers associated with the DMA channels.

- clocks: a single clock specifier.

- #dma-cells: should be <1>. The single cell represents the channel index.

Example:

	dmac: dma-controller@5a000000 {

		compatible = "socionext,uniphier-mio-dmac";

		reg = <0x5a000000 0x1000>;

		interrupts = <0 68 4>, <0 68 4>, <0 69 4>, <0 70 4>,

			     <0 71 4>, <0 72 4>, <0 73 4>, <0 74 4>;

		clocks = <&mio_clk 7>;

		#dma-cells = <1>;

	};

Note:

In the example above, "interrupts = <0 68 4>, <0 68 4>, ..." is not a typo.

The first two channels share a single interrupt line.