ARM: configs: stm32: enable ADC driver

ADC driver depends on REGULATOR and IIO that are not yet selected.
Current hardware boards (like stm32f429i-eval) is using fixed
regulators.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>