net: dsa: ar9331: provide own phylink MAC operations

Convert ar9331 to provide its own phylink MAC operations, thus
avoiding the shim layer in DSA's port.c.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/E1rvIcZ-006bQc-0W@rmk-PC.armlinux.org.uk


Signed-off-by: Paolo Abeni <pabeni@redhat.com>