net: dsa: b53: mmap: add phy ops

Implement phy_read16() and phy_write16() ops for B53 MMAP to avoid accessing
B53_PORT_MII_PAGE registers which hangs the device.
This access should be done through the MDIO Mux bus controller.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>