drivers: sensor: stm32_temp: handle disabled ADCs more gracefully

This commit modifies the STM32 internal temperature sensor driver
to handle erroneous usage more gracefully. More precisely:
 - driver no longer builds if no ADC node is enabled
 - fail builds with an explicit error message when the sensor
   is enabled but the corresponding ADC is not. This can only
   happen on STM32 series with more than one ADC (e.g., H7).

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>