octeontx2-af: Add new mbox to support multicast/mirror offload

A new mailbox is added to support offloading of multicast/mirror
functionality. The mailbox also supports dynamic updation of the
multicast/mirror list.

Signed-off-by: Suman Ghosh <sumang@marvell.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>