octeontx2-af: Forward CGX link notifications to PFs

Upon receiving notification from firmware the CGX event handler
in the AF driver gets the current link info such as status, speed,
duplex etc from CGX driver and sends it across to PFs who have
registered to receive such notifications.

To support above
 - Mbox messaging support for sending msgs from AF to PF has been added.
 - Added mbox msgs so that PFs can register/unregister for link events.
 - Link notifications are sent to PF under two scenarioss.
  1. When a asynchronous link change notification is received from
     firmware with notification flag turned on for that PF.
  2. Upon notification turn on request, the current link status is
     send to the PF.

Also added a new mailbox msg using which RVU PF/VF can retrieve
their mapped CGX LMAC's current link info. Link info includes
status, speed, duplex and lmac type.

Signed-off-by: Linu Cherian <lcherian@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>