dmaengine: renesas,usb-dmac: Add device tree bindings documentation

* Renesas USB DMA Controller Device Tree bindings

Required Properties:

- compatible: must contain "renesas,usb-dmac"

- reg: base address and length of the registers block for the DMAC

- interrupts: interrupt specifiers for the DMAC, one for each entry in

  interrupt-names.

- interrupt-names: one entry per channel, named "ch%u", where %u is the

  channel number ranging from zero to the number of channels minus one.

- clocks: a list of phandle + clock-specifier pairs.

- #dma-cells: must be <1>, the cell specifies the channel number of the DMAC

  port connected to the DMA client.

- dma-channels: number of DMA channels

Example: R8A7790 (R-Car H2) USB-DMACs

	usb_dmac0: dma-controller@e65a0000 {

		compatible = "renesas,usb-dmac";

		reg = <0 0xe65a0000 0 0x100>;

		interrupts = <0 109 IRQ_TYPE_LEVEL_HIGH

			      0 109 IRQ_TYPE_LEVEL_HIGH>;

		interrupt-names = "ch0", "ch1";

		clocks = <&mstp3_clks R8A7790_CLK_USBDMAC0>;

		#dma-cells = <1>;

		dma-channels = <2>;

	};

	usb_dmac1: dma-controller@e65b0000 {

		compatible = "renesas,usb-dmac";

		reg = <0 0xe65b0000 0 0x100>;

		interrupts = <0 110 IRQ_TYPE_LEVEL_HIGH

			      0 110 IRQ_TYPE_LEVEL_HIGH>;

		interrupt-names = "ch0", "ch1";

		clocks = <&mstp3_clks R8A7790_CLK_USBDMAC1>;

		#dma-cells = <1>;

		dma-channels = <2>;

	};