thermal/drivers/mediatek/lvts_thermal: Provision for gt variable location

The golden temperature calibration value in nvram is not always the
3rd byte. A future commit will prove this assumption wrong.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240402032729.2736685-11-nico@fluxnic.net