Documentation: dt: atmel-at91: Document secumod bindings

A security module is available starting with sama5d2, add its bindings.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/atmel-at91.txt

@@ -225,3 +225,19 @@ required properties:
 		compatible = "atmel,sama5d3-sfr", "syscon";
 		reg = <0xf0038000 0x60>;
 	};
+
+Security Module (SECUMOD)
+
+The Security Module macrocell provides all necessary secure functions to avoid
+voltage, temperature, frequency and mechanical attacks on the chip. It also
+embeds secure memories that can be scrambled
+
+required properties:
+- compatible: Should be "atmel,<chip>-secumod", "syscon".
+  <chip> can be "sama5d2".
+- reg: Should contain registers location and length
+
+	secumod@fc040000 {
+		compatible = "atmel,sama5d2-secumod", "syscon";
+		reg = <0xfc040000 0x100>;
+	};