mlxsw: reg: Add Switch Port VLAN Stacking Register

SPVTR register configures the VLAN mode of the port to enable VLAN
stacking.

It will be used to configure VxLAN to push VLAN to the decapsulated packet.
Without this setting, Spectrum-2 overtakes the VLAN tag of decapsulated
packet for bridging.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>