netlink: specs: add partial specification for openvswitch

The openvswitch family has a fixed header, uses struct attrs and has array
values. This partial spec demonstrates these features in the YNL CLI. These
specs are sufficient to create, delete and dump datapaths and to dump vports:

$ ./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/ovs_datapath.yaml \
    --do dp-new --json '{ "dp-ifindex": 0, "name": "demo", "upcall-pid": 0}'
None

$ ./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/ovs_datapath.yaml \
    --dump dp-get --json '{ "dp-ifindex": 0 }'
[{'dp-ifindex': 3,
  'masks-cache-size': 256,
  'megaflow-stats': {'cache-hits': 0,
                     'mask-hit': 0,
                     'masks': 0,
                     'pad1': 0,
                     'padding': 0},
  'name': 'test',
  'stats': {'flows': 0, 'hit': 0, 'lost': 0, 'missed': 0},
  'user-features': {'dispatch-upcall-per-cpu',
                    'tc-recirc-sharing',
                    'unaligned'}},
 {'dp-ifindex': 48,
  'masks-cache-size': 256,
  'megaflow-stats': {'cache-hits': 0,
                     'mask-hit': 0,
                     'masks': 0,
                     'pad1': 0,
                     'padding': 0},
  'name': 'demo',
  'stats': {'flows': 0, 'hit': 0, 'lost': 0, 'missed': 0},
  'user-features': set()}]

$ ./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/ovs_datapath.yaml \
    --do dp-del --json '{ "dp-ifindex": 0, "name": "demo"}'
None

$ ./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/ovs_vport.yaml \
    --dump vport-get --json '{ "dp-ifindex": 3 }'
[{'dp-ifindex': 3,
  'ifindex': 3,
  'name': 'test',
  'port-no': 0,
  'stats': {'rx-bytes': 0,
            'rx-dropped': 0,
            'rx-errors': 0,
            'rx-packets': 0,
            'tx-bytes': 0,
            'tx-dropped': 0,
            'tx-errors': 0,
            'tx-packets': 0},
  'type': 'internal',
  'upcall-pid': [0],
  'upcall-stats': {'fail': 0, 'success': 0}}]

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>