ARM: dts: stm32: enable ADC on stm32f429i-eval board

Enable analog to digital converter on stm32f429i-eval board.
It has on-board potentimeter wired to ADC3 in8 analog pin and
uses fixed regulator to provide reference voltage.

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