Documentation: devicetree: revise ethernet device-tree binding about TRGMII

add phy-mode "trgmii" to
Documentation/devicetree/bindings/net/ethernet.txt

Cc: devicetree@vger.kernel.org
Reported-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>