common/octeontx2: prevent STP instruction fissure

common/octeontx2: prevent STP instruction fissure

OTX2 AP core can sometimes fissure STP instructions when it is more
optimal to send such writes into the pipeline as 2 separate
instructions. However registers should be excluded from such
optimization. This commit ensures that no CSR write is ever fissured
by introducing zero cost workaround by setting STP pre-index by zero to
make sure OTX2 AP core prevent fissure.

Fixes: 8a4f835971f5 ("common/octeontx2: add IO handling APIs")
Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Pavan Nikhilesh <pbhagavatula@marvell.com>