ARM: dts: ixp4xx: Add a devicetree for Freecom FSG-3

This adds a devicetree for the Freecom FSG-3, a combined router
and NAS.

Cc: Rod Whitby <rod@whitby.id.au>
Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>