net: dsa: qca8k: move qca8k read/write/rmw and reg table to common code

The same reg table and read/write/rmw function are used by drivers
based on qca8k family switch.
Move them to common code to make it 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>