devlink: allow driver to update progress of flash update

Introduce a function to be called from drivers during flash. It sends
notification to userspace about flash update progress.

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