arm64: dts: mediatek: mt7981b: Add Ethernet and WiFi offload support

arm64: dts: mediatek: mt7981b: Add Ethernet and WiFi offload support

Add device tree nodes for the Ethernet subsystem on MT7981B SoC,
including:
- Ethernet MAC controller with dual GMAC support
- Wireless Ethernet Dispatch (WED)
- SGMII PHY controllers for high-speed Ethernet interfaces
- Reserved memory regions for WiFi offload processor

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
[Angelo: Removed useless address/size cells from main eth node]
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>