dt-bindings: thermal: Add documentation for CPU voltage cooling device

Add documentation for CPU voltage cooling device. CPU voltage cooling
device will provide support to apply CPU frequency mitigation on the
different CPUs in a cluster to achieve a reduction in cluster voltage.
This is achieved by building a mitigation table mapping the CPU
frequency levels to a voltage.

Change-Id: I41a222c54442bdc065267f1f1cf079118b8032e9