dt-bindings: mfd: Add Freecom system controller

This adds a DT binding for the Freecom FSG3 system controller
found at CS2 in the Freecom FSG3 Intel IXP42x-based router.

Cc: [e-mail]
Signed-off-by: Linus Walleij <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/mfd/syscon.yaml

···
               - allwinner,sun8i-v3s-system-controller
               - allwinner,sun50i-a64-system-controller
               - brcm,cru-clkset
+              - freecom,fsg-cs2-system-controller
               - hisilicon,dsa-subctrl
               - hisilicon,hi6220-sramctrl
               - hisilicon,pcie-sas-subctrl