dt-bindings: Add bindings for nvidia,tegra186-bpmp-thermal

In Tegra186, the BPMP (Boot and Power Management Processor) implements
an interface that is used to read system temperatures, including CPU
cluster and GPU temperatures. This binding describes the thermal sensor
that is exposed by BPMP.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>