of: Add descriptions of thermtrip properties to Tegra PMC bindings

Hardware-triggered thermal reset requires configuring the I2C
reset procedure. This configuration is read from the device tree,
so document the relevant properties in the binding documentation.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>