dt-bindings: arm: mediatek: document WED binding for MT7622

Document the binding for the Wireless Ethernet Dispatch core on the MT7622
SoC, which is used for Ethernet->WLAN offloading
Add related info in mediatek-net bindings.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: David S. Miller <davem@davemloft.net>