Add initial octeontx2 support.

Add initial octeontx2 support.

This adds octeontx2 naming.  It currently uses the cortexa57
cost model and schedule model until I submit this.  This is
more a place holder to get the naming of the cores in GCC 10.
I will submit the cost model in the next couple of days.

ChangeLog:
* config/aarch64/aarch64-cores.def (octeontx2): New define.
(octeontx2t98): New define.
(octeontx2t96): New define.
(octeontx2t93): New define.
(octeontx2f95): New define.
(octeontx2f95n): New define.
(octeontx2f95mm): New define.
* config/aarch64/aarch64-tune.md: Regenerate.
* doc/invoke.texi (-mcpu=): Document the new cpu types.