mlxsw: spectrum_router: Add an indication if a nexthop group can be destroyed

Currently, a nexthop group is destroyed when the last FIB entry is
detached from it.

When nexthop objects are supported, this can no longer be the case, as
the group is a separate object whose lifetime is managed by user space.

Add an indication if a nexthop group can be destroyed and always set it
to true for the existing IPv4 and IPv6 nexthop groups.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>