Documentation: arm: marvell: Add Octeon TX2 CN913x Flavors

Marvell put CN913x into Octeon TX2 family but they are different from all
other Octeon TX2 products. Instead CN913x is evolution from Armada 7k/8k
products.

Signed-off-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20210919143348.24338-1-pali@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>