dt-bindings: iio: stm32-adc: add support for STM32H7

Document support for STM32H7 Analog to Digital Converter.
Main difference is regarding compatible, clock definitions and new
features like differential channels support:
STM32H7 ADC block has two clock inputs, common clock for all ADCs.
One 'bus' clock for registers access, and one optional 'adc' clock
for analog circuitry (bus clock may be used for conversions).

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