arm64: tegra: Add PMU on Tegra210

The NVIDIA Tegra210 contains an ARM PMU v3 that can be used to gather
statistics about the processors and their memory system. Add a device
tree node so that this functionality can be exposed.

Reported-by: William Cohen <giantklein@gmail.com>
Tested-by: William Cohen <giantklein@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>