mips: dts: ralink: mt7621: add port@5 as CPU port

On MT7621AT, MT7621DAT, and MT7621ST SoCs, port 5 of the MT7530 switch is
connected to the second MAC of the SoC as a CPU port. Add the port and set
up the second MAC on the bindings. Revert PHY muxing on GB-PC1.

There's an external PHY connected to the second MAC of the SoC on GB-PC2,
therefore, disable port@5 for this device.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>