mips: dts: ralink: mt7621: mux phy4 to gmac1 for GB-PC1

Mux the MT7530 switch's phy4 to the SoC's gmac1 on the GB-PC1 devicetree.
This achieves 2 Gbps total bandwidth to the CPU using the second RGMII.

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