octeontx2-af: Allow to configure flow tag LSB byte as RSS adder

Before C0 HW revision, The RSS adder was computed based the
following static formula.

rss_adder<7:0> = flow_tag<7:0> ^ flow_tag<15:8> ^
flow_tag<23:16> ^ flow_tag<31:24>

The above scheme has the following drawbacks:
1) It is not in line with other standard NIC behavior.
2) There can be an SW use case where SW can compute the hash
upfront using Toeplitz function and predict the queue selection
to optimize some packet lookup function. The nonstandard
way of doing XOR makes the consumer to not predict the queue selection.

C0 HW revision onwards, The HW can configure the
rss_adder<7:0> as flow_tag<7:0> to align with standard NICs.

This patch adds an option to select legacy RSS adder mode
vs standard NIC behavior by setting NIX_LF_RSS_TAG_LSB_AS_ADDER flag.

Since this bit field is used as reserved in old HW revisions,
No need to have an additional HW version check.

Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>