mlxsw: reg: Add Switch Port Egress VLAN EtherType Register

SPEVET configures which EtherType to push at egress for packets incoming
through a local port for which 'SPVID.egr_et_set' is set.

The next patches will use SPEVET to configure EtherType 0x88A8 and
0x8100 for local ports member in 802.1ad and 802.1q bridges,
respectively. This allows using dual VxLAN bridges (802.1d and 802.1ad at
the same time).

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