stm32: adc: Improve handling of voltage ref and internal temp

This change includes special handling of the internal voltage
reference and internal die temperature channels for all currently
defined STM32 models

The code now looks for specific ADC + channel ID pairs instead
of just a channel ID to determine if the caller is trying to
configure an internal channel.

Signed-off-by: Pete Dietl <petedietl@gmail.com>