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>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>