octeontx2-pf: Mailbox communication with AF

In the resource virtualization unit (RVU) each of the PF and AF
(admin function) share a 64KB of reserved memory region for
communication. This patch initializes PF <=> AF mailbox IRQs,
registers handlers for processing these communication messages.
Also adds support to process these messages in both directions
ie responses to PF initiated DOWN (PF => AF) messages and AF
initiated UP messages (AF => PF).

Mbox communication APIs and message formats are defined in AF driver
(drivers/net/ethernet/marvell/octeontx2/af), mbox.h from AF driver is
included here to avoid duplication.

Signed-off-by: Geetha sowjanya <gakula@marvell.com>
Signed-off-by: Christina Jacob <cjacob@marvell.com>
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Aleksey Makarov <amakarov@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>