Add plumbing to allow DSA drivers to register parameters with devlink.

To keep with the abstraction, the DSA drivers pass the ds structure to
these helpers, and the DSA core then translates that to the devlink
structure associated to the device.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>