mlxsw: spectrum_nve: Add support for VXLAN on Spectrum-2

Spectrum-1 and Spectrum-2 are largely backward compatible with regards
to VXLAN. One difference - as explained in previous patch - is that an
underlay RIF needs to be specified instead of an underlay VR during NVE
initialization. This is accomplished by calling the relevant function
that returns the index of such a RIF based on the table ID
(RT_TABLE_MAIN) where underlay look up occurs.

The second difference is that VXLAN learning (snooping) is controlled
via a different register (TNPC).

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