dt-bindings: thermal: k3: Add VTM bindings documentation

Add VTM bindings documentation. In the Voltage Thermal
Management Module(VTM), K3 AM654 supplies a voltage
reference and a temperature sensor feature that are gathered in the band
gap voltage and temperature sensor (VBGAPTS) module. The band
gap provides current and voltage reference for its internal
circuits and other analog IP blocks. The analog-to-digital
converter (ADC) produces an output value that is proportional
to the silicon temperature.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200407055116.16082-2-j-keerthy@ti.com