dt-bindings: Document Tegra CCPLEX Cluster

The Tegra CPU COMPLEX CLUSTER area contains memory-mapped
registers that initiate CPU frequency/voltage transitions.

Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>