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

The MIO DMAC (Media IO DMA Controller) is used in UniPhier LD4,
Pro4, and sLD8 SoCs.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/uniphier-mio-dmac.txt

+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.