arm64: dts: mediatek: mt7981b-openwrt-one: Enable Ethernet

arm64: dts: mediatek: mt7981b-openwrt-one: Enable Ethernet

Enable the Ethernet subsystem on OpenWrt One board with dual-MAC
configuration:
- GMAC0: Connected to external Airoha EN8811H 2.5GbE PHY via SGMII
  (2500base-x mode) for WAN connectivity with LED indicators
- GMAC1: Connected to internal MT7981 1GbE PHY (GMII mode) for LAN

Ethernet aliases are defined to provide consistent network interface
naming (ethernet0 = LAN, ethernet1 = WAN).

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>