arch/arm/src/stm32: Added Vbat measurement to adc driver for STM32F20xx and STM32F4xxx.

As with the MCU temperature and VREFINT measurement, this patch requires user to enable the corresponding channel first. For Vbat channel the ioctl cmd is IO_ENABLE_DISABLE_VBAT_CH, and its arg should be a pointer to bool which must be true to enable and false to disable the Vbat channel.

Moreover, since Vbat input contains a built-in voltage divider, it is highly suggested to disable Vbat input channel after measurement is done in order to prevent battery drain through the divider.