mlxsw: Move devlink port registration into common core code

Remove devlink port reg/unreg from spectrum and switchx2 code and rather
do the common work in core. That also ensures code separation where
devlink is only used in core.c.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>