octeontx2-af: Add free rsrc count mbox msg

Upon receiving the MBOX_MSG_FREE_RSRC_CNT, the AF will find out the
current number of free resources and reply it back to the requester. No
guarantee is given on the future state of the free resources yet.
If another requester sends MBOX_MSG_ATTACH_RESOURCES after this call,
the number of available resources might change.

Signed-off-by: George Cherian <george.cherian@marvell.com>
Signed-off-by: Stanislaw Kardach <skardach@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>