mlxsw: spectrum_router: Create IPv6 multipath routes in one go

Allow the driver to create an IPv6 multipath route in one go by passing
an array of sibling routes and iterating over 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>