dt-bindings: net: mediatek,net: add missing mediatek,mt7621-eth

Document the Ethernet controller found in the MediaTek MT7621 MIPS SoC
family which is supported by the mtk_eth_soc driver.

Fixes: 889bcbde ("net: ethernet: mediatek: support MT7621 SoC ethernet hardware")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/ec4371c4b5a331c5217b5f13a0c9e6c444838e14.1690246066.git.daniel@makrotopia.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>