dt-bindings: net: mediatek: add support for MediaTek MT7623 and MT7622 SoC

The patch adds the supplements in the dt-binding document for MediaTek
MT7622 SoC with extra SGMII system controller and relevant clock consumers
listed as the requirements for those SoCs equipped with the SGMII circuit.
Also, add the missing binding information for MT7623 SoC here which relies
on the fallback binding of MT2701.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>