doc/netlink/specs: Add missing attr in rt_link spec

IFLA_DPLL_PIN was added to rt_link messages but not to the spec, which
breaks ynl. Add the missing definitions to the rt_link ynl spec.

Fixes: 5f184269

 ("netdev: expose DPLL pin handle for netdevice")
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240201113853.37432-1-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>