mlxsw: reg: Add Switch Port VLAN Classification Register

SPVC configures the port to identify packets as untagged / single tagged /
double tagged packets based on the packet EtherTypes.

It will be used to classify 802.1q packets as untagged and 802.1ad packets
as tagged when received by ports member in a 802.1ad bridge.

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: Jakub Kicinski <kuba@kernel.org>