dt-bindings: iio: adc: stm32: add support for diff channels
STM32H7 ADC channels may be configured either as single-ended or
differential.
Add 'st,adc-diff-channels' property to support differential channels.
Differential channels are defined as a pair of positive and negative
inputs: vinp & vinn.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com> Acked-by: Rob Herring <robh@kernel.org> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>