ARM: dts: ixp4xx: Add devicetree for D-Link DSM-G600 rev A

This adds a devicetree for the D-Link DSM-G600 Wireless Network
Storage Enclosure so that we can delete the boardfile. The boardfile
does not even define an ethernet interface as it has an external
ethernet on PCI. This devicetree is for revision A using IXP420
the rev B version uses PowerPC.

Cc: Michael-Luke Jones <mlj28@cam.ac.uk>
Cc: Rod Whitby <rod@whitby.id.au>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Michael Westerhof <mwester@dls.net>
Cc: Deepak Saxena <dsaxena@plexity.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>