arm64: tegra: Add compatible string for Tegra194 CPU complex

On Tegra194, data on valid operating points for the CPUs needs to be
queried from BPMP. However, there is no node representing CPU complex.
So, add a compatible string to the 'cpus' node instead of using dummy
node to bind the cpufreq driver to. Also, add reference to the BPMP
instance for the CPU complex.

Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>