ARM: dts: BCM5301X: Disable gmac0 and enable port@8 on Asus RT-AC88U

Disable gmac0 which is not connected to any switch MAC. Enable port@8 of
the Broadcom switch which is connected to gmac2.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>