net: dsa: qca8k: move port LAG functions to common code

The same port LAG functions are used by drivers based on qca8k family
switch. Move them to common code to make them accessible also by other
drivers.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>