doc: devicetree: Add bindings documentation for omap-des driver

OMAP SoC DES crypto Module

Required properties:

- compatible : Should contain "ti,omap4-des"

- ti,hwmods: Name of the hwmod associated with the DES module

- reg : Offset and length of the register set for the module

- interrupts : the interrupt-specifier for the DES module

- clocks : A phandle to the functional clock node of the DES module

           corresponding to each entry in clock-names

- clock-names : Name of the functional clock, should be "fck"

Optional properties:

- dmas: DMA specifiers for tx and rx dma. See the DMA client binding,

	Documentation/devicetree/bindings/dma/dma.txt

	Each entry corresponds to an entry in dma-names

- dma-names: DMA request names should include "tx" and "rx" if present

Example:

	/* DRA7xx SoC */

	des: des@480a5000 {

		compatible = "ti,omap4-des";

		ti,hwmods = "des";

		reg = <0x480a5000 0xa0>;

		interrupts = <GIC_SPI 82 IRQ_TYPE_LEVEL_HIGH>;

		dmas = <&sdma 117>, <&sdma 116>;

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

		clocks = <&l3_iclk_div>;

		clock-names = "fck";

	};