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

This patch adds documentation for Device-Tree bindings for the
Security System cryptographic accelerator driver.
Signed-off-by: NLABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/crypto/sun4i-ss.txt

+* 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";
+	};