mlxsw: spectrum_nve: Implement common NVE core

The Spectrum ASIC supports different types of NVE encapsulations (e.g.,
VxLAN, NVGRE) with more types to be supported by future ASICs.

Despite being different, all these encapsulations share some common
functionality such as the enablement of NVE encapsulation on a given
filtering identifier (FID) and the addition of remote VTEPs to the
linked-list of VTEPs that traffic should be flooded to.

Implement this common core and allow different ASICs to register
different operations for different encapsulation types.

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