ARM: dts: BCM5301X: Add rgmii to port@5 of Broadcom switch on Asus RT-AC88U

Define phy-mode of the Broadcom switch's port@5 as rgmii. This doesn't seem
to matter but let's explicitly define it since phy-mode as rgmii is defined
on the other side which is port@6 of the Realtek switch.

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