drivers: adc: stm32: enable battery voltage input without CONFIG_STM32_VBAT

Allow the battery voltage to be read through the ADC even if the
CONFIG_STM32_VBAT driver is not enabled.

I guessed this part of code is conditionally compiled depending on
CONFIG_STM32_VBAT because this feature, and hence the
LL_ADC_CHANNEL_VBAT constant is not available on all families. As the
feature is already checked at runtime (as the same driver supports
multiple instances), we can conditionally compile it depending on
LL_ADC_CHANNEL_VBAT instead.

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