dt-bindings: net: mediatek: update documentation for reset signals

Since there's no user for the property reset inside the ethernet node
for current supported MediaTek SoCs and boards, so it should be safe to
update reset property in the bindings to introduce more reset signals as
corresponding that commit 7c2adaf1

 ("reset: mediatek: Add MT2701 ethsys
reset controller include file") did in order to be referenced them from
within a devicetree file.

Cc: John Crispin <john@phrozen.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>