netlink-specs: add partial specification for devlink

Devlink is quite complex but put in the very basics so we can
incrementally fill in the commands as needed.

$ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/devlink.yaml \
    --dump get

[{'bus-name': 'netdevsim',
  'dev-name': 'netdevsim1',
  'dev-stats': {'reload-stats': {'reload-action-info': {'reload-action': 1,
                                                        'reload-action-stats': {'reload-stats-entry': [{'reload-stats-limit': 0,
                                                                                                        'reload-stats-value': 0}]}}},
                'remote-reload-stats': {'reload-action-info': {'reload-action': 2,
                                                               'reload-action-stats': {'reload-stats-entry': [{'reload-stats-limit': 0,
                                                                                                               'reload-stats-value': 0},
                                                                                                              {'reload-stats-limit': 1,
                                                                                                               'reload-stats-value': 0}]}}}},
  'reload-failed': 0}]

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>