octeon_ep: add PF-VF mailbox communication

Implement mailbox communication between PF and VFs.
PF-VF mailbox is used for all control commands from VF to PF and
asynchronous notification messages from PF to VF.

Signed-off-by: Shinas Rasheed <srasheed@marvell.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>