nfp: add very basic access to NSP logs

Allow dumping "arm.diag" resource with ethtool -w.  This resource
should contain a text log of the NSP (control processor) application.

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