dts: mtmips: add default pinctrl to eth nodes for all boards

This patch adds default eth pinctrl for all boards.

There are two pinctrl nodes used for two scenarios:
ephy_iot_mode    - for IOT boards which have only one port (PHY0)
ephy_router_mode - For routers which have more than one ports

Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/arch/mips/dts/linkit-smart-7688.dts b/arch/mips/dts/linkit-smart-7688.dts
index 740b73b..f154aae 100644
--- a/arch/mips/dts/linkit-smart-7688.dts
+++ b/arch/mips/dts/linkit-smart-7688.dts
@@ -53,5 +53,7 @@
 };
 
 &eth {
+	pinctrl-names = "default";
+	pinctrl-0 = <&ephy_iot_mode>;
 	mediatek,poll-link-phy = <0>;
 };