mlxsw: spectrum_nve: Implement VxLAN operations

The common NVE core expects each encapsulation type to implement a
certain set of operations that are specific to this type and the
currently used ASIC. These operations include things such as the ability
to determine whether a certain NVE configuration can be offloaded and
ASIC-specific initialization for this type.

Implement these operations for VxLAN on the Spectrum ASIC. Spectrum-2
support will be added by a future patchset.

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