drivers: sensor: stm32_temp/vbat/vref: ensure the ADC is not suspended

The STM32 temperature, battery and voltage reference sensors needs to
write the ADC common registers to enable the corresponding ADC features.
For that we need to ensure that the ADC is not suspended by using the
pm_device_runtime_get/put methods.

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