mfd: syscon: Update Atmel SMC binding doc

A new compatible string is introduced for SMC on sama5d2 to manage a
different layout of the registers.
Signed-off-by: NLudovic Desroches <ludovic.desroches@microchip.com>
│Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/atmel-smc.txt

@@ -8,6 +8,7 @@ Required properties:
 - compatible:		Should be one of the following
 			"atmel,at91sam9260-smc", "syscon"
 			"atmel,sama5d3-smc", "syscon"
+			"atmel,sama5d2-smc", "syscon"
 - reg:			Contains offset/length value of the SMC memory
 			region.