net: dsa: move rest of devlink setup/teardown to devlink.c

The code that needed further refactoring into dedicated functions in
dsa2.c was left aside. Move it now to devlink.c, and make dsa2.c stop
including net/devlink.h.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>