mlxsw: spectrum_router: Add virtual router management

Virtual router is a construct used inside HW. In this implementation
we map kernel tables to virtual routers one to one. Introduce management
logic to create virtual routers when needed and destroy in case they are
no longer in use. According to that, call into LPM tree management.
Each virtual router is always bound to one LPM tree.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>