Add description of Cavium Octeon and ThunderX SOC device tree bindings.

CC: Ulf Hansson <ulf.hansson@linaro.org>
CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: devicetree@vger.kernel.org
Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Steven J. Hill <steven.hill@cavium.com>
Acked-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>