mlxsw: reg: Add Switch Virtual-Port Enabling register definition

In order for a port to support {Port, VID} to FID mapping it needs to be
configured to a virtual port mode (as opposed to VLAN mode).

Add the SVPE register, which enables port virtualization.

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>