net: dsa: qca8k: move qca8k bulk read/write helper to common code

The same ATU function are used by drivers based on qca8k family switch.
Move the bulk read/write helper to common code to declare these shared
ATU functions in common code.
These helper will be dropped when regmap correctly support bulk
read/write.

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