nfp: add support for per-PCI PF mailbox

When working with devlink-related functionality for locking reasons
it's easier to create a new mailbox per-PCI PF device than try to
use one of the netdev/vNIC mailboxes.

Define new mailbox structure and resolve its symbol during probe.
For forward compatibility allow silent truncation of mailbox command
data.

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