ARM: sun4i: dt: Add DT bindings documentation for SUN4I Security System

* Allwinner Security System found on A20 SoC

Required properties:

- compatible : Should be "allwinner,sun4i-a10-crypto".

- reg: Should contain the Security System register location and length.

- interrupts: Should contain the IRQ line for the Security System.

- clocks : List of clock specifiers, corresponding to ahb and ss.

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

	* "ahb" : AHB gating clock

	* "mod" : SS controller clock

Example:

	crypto: crypto-engine@01c15000 {

		compatible = "allwinner,sun4i-a10-crypto";

		reg = <0x01c15000 0x1000>;

		interrupts = <GIC_SPI 86 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&ahb_gates 5>, <&ss_clk>;

		clock-names = "ahb", "mod";

	};