net: dsa: b53: Add BCM5389 support

This patch adds support for the BCM5389 switch connected through MDIO.

Signed-off-by: Damien Thébault <damien.thebault@vitec.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>