dt-bindings: atmel-sysreg: add microchip,sam9x60-ddramc

Add microchip,sam9x60-ddramc 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/1578673089-3484-10-git-send-email-claudiu.beznea@microchip.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

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

@@ -39,6 +39,7 @@ RAMC SDRAM/DDR Controller required properties:
 			"atmel,at91sam9260-sdramc",
 			"atmel,at91sam9g45-ddramc",
 			"atmel,sama5d3-ddramc",
+			"microchip,sam9x60-ddramc"
 - reg: Should contain registers location and length
 
 Examples: