devlink: add device information API

ethtool -i has served us well for a long time, but its showing
its limitations more and more. The device information should
also be reported per device not per-netdev.

Lay foundation for a simple devlink-based way of reading device
info. Add driver name and device serial number as initial pieces
of information exposed via this new API.

v3:
 - rename helpers (Jiri);
 - rename driver name attr (Jiri);
 - remove double spacing in commit message (Jiri).
RFC v2:
 - wrap the skb into an opaque structure (Jiri);
 - allow the serial number of be any length (Jiri & Andrew);
 - add driver name (Jonathan).

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>