mips: dts: ralink: mt7621: fix external phy on GB-PC2

The address of the external phy on the mdio bus is 5. Update the devicetree
for GB-PC2 accordingly.

Fixes: 5bc14864 ("staging: mt7621-dts: fix GB-PC2 devicetree")
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>