netlink: specs: Add a spec for neighbor tables in rtnetlink

Add a YNL spec for neighbour tables and neighbour entries in rtnetlink.

./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/rt_neigh.yaml \
    --dump getneigh
[{'cacheinfo': {'confirmed': 122664055,
                'refcnt': 0,
                'updated': 122658055,
                'used': 122658055},
  'dst': '0.0.0.0',
  'family': 2,
  'flags': set(),
  'ifindex': 5,
  'lladr': '',
  'probes': 0,
  'state': {'noarp'},
  'type': 'broadcast'},
  ...]

Acked-by: Stanislav Fomichev <[email protected]>
Signed-off-by: Donald Hunter <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: donaldh