octeontx2-af: nix and lbk in loop mode in 98xx

In 98xx, there are 2 NIX blocks and 4 LBK blocks present. The way
these NIX-LBK should be configured depends on the use case. By
default loopback functionality is supported in AF VF pairs which
are attached to NIX0 and NIX1 LFs alternatively to ensure load
balancing. NIX0 transmits a packet to LBK1 which will be received
by NIX1 and packet transmitted by NIX1 will get received by NIX0 via
LBK2.

There are some requirements where only one AF VF is used and respective
NIX is expected to operate in a mode where it can receive it own packet
back. This can be achieved if NIX0 sends packet to LBK0 and not LBK1.
Adding a flag in LF alloc request mailbox which can setup NIX0 to use
LBK0 and NIX1 can use LBK3.

Signed-off-by: Harman Kalra <hkalra@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>