dt-bindings: arm: Add NVIDIA Tegra194 CPU Complex binding

Add device-tree binding documentation to represent Tegra194
CPU Complex with compatible string under 'cpus' node. This
can be used by drivers like cpufreq which don't have their
node or CPU Complex node to bind to. Also, documenting
'nvidia,bpmp' property which points to BPMP device.

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