arm: dts: mt7623: add MT7623N reference board with eMMC

Add MT7623N reference board with eMMC. On the board, there is additional
external PHY ICPlus IP1001 transceiver available by port 5 on the MDIO
bus connectted with GMAC2.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Suggested-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>