dt-bindings: arm: bcm: add bindings for ASUS RT-AC3200

Add ASUS RT-AC3200 under BCM4709 based boards.

Acked-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Arınç ÜNAL <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Florian Fainelli <[e-mail]>

Documentation/devicetree/bindings/arm/bcm/brcm,bcm4708.yaml

···
       - description: BCM4709 based boards
         items:
           - enum:
+              - asus,rt-ac3200
               - asus,rt-ac87u
               - buffalo,wxr-1900dhp
               - linksys,ea9200