adc: Add driver for STM32 ADC

This patch adds support for STMicroelectronics STM32 ADC (analog to
digital converter). It's originally based on Linux kernel v4.18-rcs
drivers/iio/adc/stm32-adc*. It's composed of:
- core driver (UCLASS_SIMPLE_BUS) manages common resources (clk, regu).
- child drivers (UCLASS_ADC) declare each ADC, channels and handle
  conversions.
This driver currently supports STM32H7 and STM32MP1 ADC.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>