thermal: ti-soc-thermal: use standard GPIO DT bindings
This change updates the ti-soc-thermal driver to use
standard GPIO DT bindings to read the GPIO number associated
to thermal shutdown IRQ, in case the device features it.
Previously, the code was using a specific DT bindings.
As now OMAP supports the standard way to model GPIOs,
there is no point in having a ti specific binding.