Add documentation for Cavium's ThunderX2 CN99XX ARM64 processor. This
SoC will use "cavium,thunderx2-cn9900" as the compatible property.

Also add a documentation entry for the "cavium,thunder2" cpu core
present in these SoCs.
Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>