arm64: dts: broadcom: bcm4908: describe internal switch

BCM4908 has internal switch with 5 GPHYs. Ports 0 - 3 are always
connected to the internal PHYs. Remaining ports depend on device setup.

Asus GT-AC5300 has an extra switch with its PHYs accessible using the
internal MDIO.

CPU port and Ethernet interface remain to be documented.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>