MAINTAINERS: Broadcom Vulcan is now Cavium ThunderX2

Now that the dts files are moved to the new location, we can remove
the Broadcom Vulcan entry in the MAINTAINERS.

Also fix up the Cavium ThunderX2 entry with the correct pattern for
the new names.

Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>