mlxsw: spectrum: Introduce Port-VLAN structure

This is the first step in the transition from the vPort model to a
unified Port-VLAN structure. The new structure is defined and created /
destroyed upon invocation of the 8021q ndos, but it's not actually used
throughout the code.

Subsequent patches will initialize it correctly and also create /
destroy it upon switchdev's VLAN object.

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