Update mvneta/e6000sw for new DSA Device Tree Bindings

In the latest Linux kernel revisions the DSA (Distributed
Switch Architecture) device tree binding was changed.
Instead of the top level dsa@ node, the switch and its
ports is represented as a child node of the mdio bus.
With that other modifications were added, such as
relation with the ethernet port of the SoC. Adjust
e6000sw etherswitch and mvneta drivers to that.

Tested on Armada 3720 EspressoBin and Armada 388 Clearfog Pro boards.

Submitted by: Bert JW Regeer <xistence@0x58.com>
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D19036