nfp: read mailbox address from TLV caps

Allow specifying alternative vNIC mailbox location in TLV caps.
This way we can size the mailbox to the needs and not necessarily
waste 512B of ctrl memory space.

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>