iio: adc: stm32-adc: add missing vdda-supply

Add missing vdda-supply, analog power supply, to STM32 ADC. When vdda is
an independent supply, it needs to be properly turned on or off to supply
the ADC.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Fixes: 1add6988

 ("iio: adc: Add support for STM32 ADC core").
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>