arm64: dts: broadcom: bcm4908: set Asus GT-AC5300 port 7 PHY mode

Port 7 is connected to the external BCM53134S switch using RGMII.

Fixes: 527a3ac9 ("arm64: dts: broadcom: bcm4908: describe internal switch")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>