dt-bindings: net: mediatek: add WED RX binding for MT7981 eth driver

Add compatible string for mediatek,mt7981-wed as MT7981 also supports
RX WED just like MT7986, but needs a different firmware file.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>