octeon_ep: Add mailbox for control commands

Add mailbox between host and NIC to send control commands from host to
NIC and receive responses and notifications from NIC to host driver,
like link status update.

Signed-off-by: Veerasenareddy Burru <vburru@marvell.com>
Signed-off-by: Abhijit Ayarekar <aayarekar@marvell.com>
Signed-off-by: Satananda Burla <sburla@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>