MAINTAINERS: Update Cavium ThunderX2 entry

Add Robert Richter as the primary maintainer for this platform.

Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Olof Johansson <olof@lixom.net>