arm: dts: mediatek: add support for all three GMACs for mt7988

This patch add all three GMACs nodes for mt7988. Each GMAC can be
configured to connect to different ethernet switches/PHYs.

diff --git a/arch/arm/dts/mt7988-rfb.dts b/arch/arm/dts/mt7988-rfb.dts
index f013e3e..817fe40 100644
--- a/arch/arm/dts/mt7988-rfb.dts
+++ b/arch/arm/dts/mt7988-rfb.dts
@@ -50,9 +50,8 @@
 	status = "okay";
 };
 
-&eth {
+&eth0 {
 	status = "okay";
-	mediatek,gmac-id = <0>;
 	phy-mode = "usxgmii";
 	mediatek,switch = "mt7988";