dt-bindings: net: mediatek: add WED RX binding for MT7986 eth driver

Document the binding for the RX Wireless Ethernet Dispatch core on the
MT7986 ethernet driver used to offload traffic received by WLAN NIC and
forwarded to LAN/WAN one.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>