crypto: cavium/nitrox - Enabled Mailbox support

Enabled the PF->VF Mailbox support. Mailbox message are interpreted
as {type, opcode, data}. Supported message types are REQ, ACK and NACK.

Signed-off-by: Srikanth Jampala <Jampala.Srikanth@cavium.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>