octeon_ep: add separate mailbox command and response queues

Enhance control mailbox protocol to support following
 - separate command and response queues
    * command queue to send control commands to firmware.
    * response queue to receive responses and notifications from
      firmware.
 - variable size messages using scatter/gather

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