dt-bindings: iio: dac: stm32-dac: convert bindings to json-schema

Convert the STM32 DAC binding to DT schema format using json-schema

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>