mlxsw: spectrum_router: Add / delete multiple IPv6 nexthops

Currently, the functions that take care of populating IPv6 nexthop
groups only add / delete a single nexthop.

Prepare them to handle multiple routes in one notification by passing an
array of routes and adding / deleting all of them.

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