Documentation: net: octeontx2: Add RVU HW and drivers overview

Added high level overview of OcteonTx2 RVU HW and functionality of
various drivers which will be upstreamed.

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