dt-bindings: atmel-smc: add microchip,sam9x60-smc

Add microchip,sam9x60-smc to DT bindings documentation.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1578673089-3484-11-git-send-email-claudiu.beznea@microchip.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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

@@ -9,6 +9,7 @@ Required properties:
 			"atmel,at91sam9260-smc", "syscon"
 			"atmel,sama5d3-smc", "syscon"
 			"atmel,sama5d2-smc", "syscon"
+			"microchip,sam9x60-smc", "syscon"
 - reg:			Contains offset/length value of the SMC memory
 			region.