nfp: add support for sending control messages via mailbox

FW may prefer to handle some communication via a mailbox
or the vNIC may simply not have a control queue (VFs).
Add a way of exchanging ccm-compatible messages via a
mailbox.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>