MAINTAINERS: update Cavium ThunderX2 maintainers

MAINTAINERS: update Cavium ThunderX2 maintainers

jnair is no longer at caviumnetworks.com (or at marvell.com). This also
means that Cavium ThunderX2 will now be maintained by Robert.

This is probably a good time to map various email addresses used for
my patches to my personal email ID, update .mailmap to do this.

Link: https://lore.kernel.org/r/20191106035203.5389-1-c.jayachandran@gmail.com
Signed-off-by: Jayachandran C <c.jayachandran@gmail.com>
Acked-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Olof Johansson <olof@lixom.net>