net: ethernet: mediatek: add the dts property to set if TRGMII supported on GMAC0

Add the dts property for the capability if TRGMII supported on GAMC0
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/mediatek-net.txt

@@ -31,7 +31,10 @@ Optional properties:
 Required properties:
 - compatible: Should be "mediatek,eth-mac"
 - reg: The number of the MAC
-- phy-handle: see ethernet.txt file in the same directory.
+- phy-handle: see ethernet.txt file in the same directory and
+	the phy-mode "trgmii" required being provided when reg
+	is equal to 0 and the MAC uses fixed-link to connect
+	with inernal switch such as MT7530.
 
 Example: