dmaengine: Add binding document for IMG MDC

* IMG Multi-threaded DMA Controller (MDC)

Required properties:

- compatible: Must be "img,pistachio-mdc-dma".

- reg: Must contain the base address and length of the MDC registers.

- interrupts: Must contain all the per-channel DMA interrupts.

- clocks: Must contain an entry for each entry in clock-names.

  See ../clock/clock-bindings.txt for details.

- clock-names: Must include the following entries:

  - sys: MDC system interface clock.

- img,cr-periph: Must contain a phandle to the peripheral control syscon

  node which contains the DMA request to channel mapping registers.

- img,max-burst-multiplier: Must be the maximum supported burst size multiplier.

  The maximum burst size is this value multiplied by the hardware-reported bus

  width.

- #dma-cells: Must be 3:

  - The first cell is the peripheral's DMA request line.

  - The second cell is a bitmap specifying to which channels the DMA request

    line may be mapped (i.e. bit N set indicates channel N is usable).

  - The third cell is the thread ID to be used by the channel.

Optional properties:

- dma-channels: Number of supported DMA channels, up to 32.  If not specified

  the number reported by the hardware is used.

Example:

mdc: dma-controller@18143000 {

	compatible = "img,pistachio-mdc-dma";

	reg = <0x18143000 0x1000>;

	interrupts = <GIC_SHARED 27 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 28 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 29 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 30 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 31 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 32 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 33 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 34 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 35 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 36 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 37 IRQ_TYPE_LEVEL_HIGH>,

		     <GIC_SHARED 38 IRQ_TYPE_LEVEL_HIGH>;

	clocks = <&system_clk>;

	clock-names = "sys";

	img,max-burst-multiplier = <16>;

	img,cr-periph = <&cr_periph>;

	#dma-cells = <3>;

};

spi@18100f00 {

	...

	dmas = <&mdc 9 0xffffffff 0>, <&mdc 10 0xffffffff 0>;

	dma-names = "tx", "rx";

	...

};