realtek: D-Link make common DT include generic

realtek: D-Link make common DT include generic

The D-Link DGS device tree was reorganized to better reflect the common
DT parts. The common include is named SOC specific (838X) and it seemed
like a good choice to add another common include in the future for the
RTL839X devices. From the current point of view this option is not really
needed.

1. The common part only includes data that matches RTL839X devices too.
2. The Panasonic DT structure avoids including the basic DTSI inside the
   common DTSI.

Taking simplicity of the Panasonic include logic and in perparation to
provide DGS-1210-52 support it makes sense to harmonize this.

- rename common include to reflect its content
- move the link to the root DTSI directly to the device specific DTS

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>