devlink: introduce a helper to generate physical port names

Each driver implements physical port name generation by itself. However
as devlink has all needed info, it can easily do the job for all its
users. So implement this helper in devlink.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>