net: dsa: qca8k: Allow overwriting CPU port setting

Implement adjust_link function that allows to overwrite default CPU port
setting using fixed-link device tree subnode.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>