mlxsw: spectrum_router: Extend virtual routers with IPv6 support

A Virtual Router (VR) is an entity which corresponds to a VRF and
performs FIB lookup in an LPM tree according to the {VR, IP Proto} ->
Tree binding.

Extend the virtual router data structure towards IPv6 FIB offload.

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>