drivers: sensor: stm32_temp: use the ADC reference voltage

The stm32_temp driver defines the ts-voltage-mv property to determine
the reference voltage of the ADC in the temperature computation. However
this information is already available in the device tree at the ADC
level (even with the same default value). Use it through the ADC API
instead of duplicating the information.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>