dt-bindings: iio: stm32-adc: add support for STM32MP1

Document support for STM32MP1 ADC. It's quite similar to STM32H7 ADC.
Introduce "st,stm32mp1-adc" compatible to handle variants of this
hardware such as vregready flag, interrupts, clock rate.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>