net: dsa: mv88e6xxx: Export VTU as devlink region

Export the raw VTU data and related registers 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>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201109082927.8684-1-tobias@waldekranz.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>