bnxt: Move generic devlink code to new file

Moving generic devlink code (registration) out of VF-R code
into new bnxt_devlink file, in preparation for future work
to add additional devlink functionality to bnxt.

Signed-off-by: Steve Lin <steven.lin1@broadcom.com>
Acked-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>