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.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>