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>

MAINTAINERS

@@ -3176,6 +3176,7 @@ S:	Supported
 F:	drivers/crypto/cavium/cpt/
 
 CAVIUM THUNDERX2 ARM64 SOC
+M:	Robert Richter <rrichter@cavium.com>
 M:	Jayachandran C <jnair@caviumnetworks.com>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained