Documentation: devicetree: change the mediatek ethernet compatible string

When the binding was defined, I was not aware that mt2701 was an earlier
version of the SoC. For sake of consistency, the ethernet driver should
use mt2701 inside the compat string as this is the earliest SoC with the
ethernet core.

The ethernet driver is currently of no real use until we finish and
upstream the DSA driver. There are no users of this binding yet. It should
be safe to fix this now before it is too late and we need to provide
backward compatibility for the mt7623-eth compat string.

Reported-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: John Crispin <john@phrozen.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>