octeontx2-af: Mbox communication support btw AF and it's VFs

VFs attached to PFs other than AF can not communicate with AF
directly. Instead they are supposed to first send message to
the PF they are residing on and PF forwards it to the AF.
Responses to messages are handled in the reverse order.

On the other hand if VFs are on AF (PF0) itself then direct mailbox
communication is possible since there's no other PF in the way.

This patch addresses this particular case and adds support for
handling it.

Signed-off-by: Tomasz Duszynski <tduszynski@marvell.com>
Signed-off-by: Marko Kallio <mkallio@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>