crypto: sunxi-ss - Document optional reset control bindings

Later Allwinner SoCs split out the reset controls for individual modules
out of the clock gate controls. The "Security System" crypto engine is
no different.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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

@@ -9,6 +9,10 @@ Required properties:
 	* "ahb" : AHB gating clock
 	* "mod" : SS controller clock
 
+Optional properties:
+ - resets : phandle + reset specifier pair
+ - reset-names : must contain "ahb"
+
 Example:
 	crypto: crypto-engine@01c15000 {
 		compatible = "allwinner,sun4i-a10-crypto";