iio: adc: stm32: add support for STM32H7

Add support for STM32H7 Analog to Digital Converter. It has up
to 20 external channels, resolution ranges from 8 to 16bits.
Either bus or asynchronous adc clock may be used.

Add registers & bitfields definition. Also add new configuration
options to enter/exit powerdown and perform self-calibration.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>