net: dsa: mv88e6xxx: Export STU as devlink region

Export the raw STU data in a devlink region so that it can be
inspected from userspace and compared to the current bridge
configuration.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>