ARM: at91: Add Special Function Registers binding documentation

The special function registers gather some registers that allow to tweak
features provided by IPs controlled through another register range.
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
[nicolas.ferre@atmel.com: reg size: 0x60]
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>

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

@@ -136,3 +136,19 @@ Example:
 		compatible = "atmel,at91sam9260-rstc";
 		reg = <0xfffffd00 0x10>;
 	};
+
+Special Function Registers (SFR)
+
+Special Function Registers (SFR) manage specific aspects of the integrated
+memory, bridge implementations, processor and other functionality not controlled
+elsewhere.
+
+required properties:
+- compatible: Should be "atmel,<chip>-sfr", "syscon".
+  <chip> can be "sama5d3" or "sama5d4".
+- reg: Should contain registers location and length
+
+	sfr@f0038000 {
+		compatible = "atmel,sama5d3-sfr", "syscon";
+		reg = <0xf0038000 0x60>;
+	};