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: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211122102210.3137559-1-linus.walleij@linaro.org